The Virus

Although viruses have probably been around since life on this planet began, they were discovered only a scant hundred years ago. In 1892 a Russian botanist, Dmitry Ivanovsky, was investigating tobacco mosaic disease, which caused mottling and blistering of tobacco leaves. Ivanovsky passed the sap of infected plants through a porcelain filter that was believed to trap all types of microorganisms, including bacteria, the smallest known pathogens. The filter had been invented by Charles Chamberlain, the trusted assistant of Louis Pasteur, and was used in many homes to filter and purify drinking water.

Surprisingly, however, the filtered sap still caused infection. Obviously the infectious agent was making it through the filter. Could the disease be caused by a poison or toxin made by the bacteria? Certain human diseases such as diphtheria were indeed caused by potent bacterial toxins that would easily pass through Chamberlain filters. Many scientists believed this to be the case. Alas, they were mistaken. A half dozen years later another botanist, a Dutchman named Martinus Beijerinck, continued the investigation. He took the sap from an infected plant, sprayed it on a healthy plant, took the sap from that plant after it became diseased, and in this manner passed the sap through several plant generations. Through all the successive inoculations plants continued to develop the characteristic mottling of tobacco mosaic disease. The sap retained its full virulence after any number of plant passages. This non-dilutable property suggested an entity that was reproducing itself within the plant. So much for the idea of toxins. It wasn't a bacterium, and it wasn't a toxin. At first Beijerinck called the infectious agent a contagium vivium fluidum-a "contagious living fluid." Later he coined the term virus, which is Latin for "poison" or "poisonous slime." Still, no one really knew what they were dealing with.

Lack of knowledge about the nature of viruses notwithstanding, soon other diseases were found to be caused by this "soluble living germ." Foot-and-mouth disease of cattle was the first animal illness shown to be transmitted by a filterable agent smaller than any known bacterium. By 1900 a human disease, yellow fever, was proved to be of viral origin. Today we know of thousands of different viruses, and they infect everything that is alive.

How do viruses work?

Viruses are not super tiny bacteria. Viruses and bacteria are in fact, so fundamentally different that in many respects a bacterium is more closely related to a human than to a virus. At least humans and bacteria are both made of cells. All living things are cellular except viruses. The structures within a cell necessary to perform the life activities of eating, energy production, growth, and response to environmental change are absent in a virus. A virus is, in fact, nothing more than a tiny, lifeless, totally inert particle-as long as it remains outside the cell. Tragically it is made to get inside the cell. Once inside, the deadly game of viral infection begins.

All viruses consist of two parts: a nucleic acid core and a protein coat surrounding the core. In some cases there is an additional fatty, or lipid, envelope. It is the function of the protein coat and lipid envelope (if present) to attach the viral particle to the cell membrane and -somehow- get the virus into the cell. This is not easy. The surface of the viral coat must fit exactly into "receptor" sites on the cell membrane. If the fit is not precise, then attachment and subsequent penetration into the cell cannot occur. Even in an ideal match up probably only one out of every few thousand encounters between virus and suitable cell results in proper binding of the two. The exactness of fit necessary for viral binding or attachment explains why viruses are usually species-specific - they will not infect cells of totally different species. Notable exceptions are the rabies and influenza viruses, both of which have a wide range of hosts.

Not only will viruses seldom infect totally different species, but quite often they are specific to particular types of cells within an organism. The hepatitis B virus targets liver cells. HIV goes for particular binding sites, or markers, on certain white blood cells. Viruses that cause the common cold bind to cells lining the respiratory tract.

Once the virus is attached to the cell, it can penetrate the membrane and enter the cell in a number of ways. It may cause the cell membrane to infold and pinch off a tiny vesicle with the virus inside. Viruses with fatty envelopes may fuse their envelopes with the cell membrane, penetrating it and letting the rest of the virus into the cell.

By whatever means, when a virus gets inside the cell, it can do one of several things. What it does determines the course of the disease. In many cases the virus begins replicating itself immediately. Reproduction and wide-spread distribution are, after all, a virus's raison d'etre. To accomplish this the virus commandeers the machinery of the cell, coercing it into churning out more viral particles. It is an incredible act of piracy that is orchestrated and controlled by the virus's nucleic acid.

Nucleic acid is a remarkable substance whose biological importance was not realized until 1953. It was in that year that James Watson and Francis Crick determined the molecular structure of DNA, one type of nucleic acid. There is another type, called RNA.

DNA is the substance of genes-the genetic material. Found in every living cell (except mature red blood cells), it is what makes a tree a tree, a bacterium a bacterium, and a human a human. It is what makes one human different from another. Structurally speaking, DNA can be viewed as a very, very long, microscopic charm bracelet (three billion charms in the case of humans). More accurately, it is two charm bracelets, lined up so that their charms bond to one another. There are four different kinds of charms, called nitrogen bases and designated by the letters A, T, C, and G. The double chain also has an obvious twist, giving it the overall look of a twisted rope ladder (a double helix), with the rungs representing the paired nitrogen bases.

Like a four-letter alphabet, the sequence of rungs or base pairs spells out what a cell will do and become. It is a blueprint for life, controlling the functioning of a cell by determining the proteins that the cell makes. But DNA does not work alone. It does not, in fact, directly control protein synthesis, which occurs on tiny particles, or protein factories, that float around the cell. DNA lies buried deep within the nucleus, interwoven into the fabric of the chromosomes. It is too valuable to go traipsing around the cell. Instead, it makes a messenger strand of nucleic acid using its base pair sequence as a template. This messenger molecule is RNA, and it carries the code from the DNA to the protein factories outside the nucleus.

Like living cells, viruses contain nucleic acid as their genetic material. A particular virus may contain DNA or RNA but not both. Unlike the DNA of the cell, however, viral nucleic acid codes not for cellular proteins but for proteins necessary to make more viruses. And it forces the cell to do just that-churn out proteins that the virus needs to make more of itself. It is an ingenious multi-phased takeover that goes something like this:

• Proteins are produced that are predominantly enzymatic in nature. They encourage reactions that produce many thousands of copies of viral nucleic acid.
• After viral nucleic acid is synthesized, the structural coat proteins are produced.
• The virus is assembled by having the coat protein form as a shell around the nucleic acid core.
• These new viral particles are released, sometimes-but not always-killing the cell in the process. In some instances the virus grabs a snippet of cell membrane, which becomes its lipid envelope.

The precise mechanism by which viruses initiate their replication depends on the genetic material of the virus. A DNA virus has a mode of action very similar to host DNA. First, a viral RNA is produced using the virus's DNA as a template. Then the RNA directs production of viral proteins. It can be summarized as follows: viral DNA - viral RNA - viral protein.

With most RNA viruses the first step is eliminated and the viral nucleic acid simply proceeds to make the proper viral proteins: viral RNA - viral protein.

The concept that DNA begets RNA, which begets protein is so basic to biology that it has come to be known as the central dogma. Never does RNA produce DNA. Never, that is, except in the case of a retrovirus. This class of RNA viruses, which counts HIV, the AIDS virus, among its members, startled molecular biologists by doing the unthinkable-by working backward. Instead of making protein, the retrovirus's RNA first goes in reverse and acts as a template for DNA synthesis. The DNA then makes another RNA, which finally goes about churning out protein molecules: viral RNA - viral DNA - viral RNA - viral protein.

Origin of viruses

It appears that somehow, some way, viruses will find a means to gain biological profit at our expense. As the historical record shows, they have been doing so for thousands of years. The pockmarked mummified face of Ramses V is mute testimony to the presence of smallpox in ancient Egypt. Bas-relief hieroglyphics from that period also depict a priest with shriveled legs, suggestive of polio. And without question viral infection predates ancient Egypt. Almost certainly the earliest cave dwellers had viruses to contend with. The ways in which viruses slip into cells and seize control could only come about through a very long association in which viruses have adapted and are continually adapting to their hosts.

Exactly when the association began is still a matter of conjecture. One theory proposes that viruses, because they are simpler than cells, actually arrived on the scene first. According to this hypothesis, nucleic acids increased in complexity until they became the stuff of cells. Although once popular, this theory is now considered unlikely. A more probable scenario is that viruses have evolved from bits of cellular genetic material that escaped from their cells. Over time, according to the "escaped gene" hypothesis, these scraps of host-derived nucleic acid developed the ability to be independent, self-replicating, intracellular parasites. They became viruses.

One scientist, the noted British astronomer Fred Hoyle, has even proposed that viruses originally fell, and continue to fall, from outer space. Not exactly a consensus opinion among today's virologists. Whatever their origin, once viruses established themselves on this planet they were a force to be reckoned with.

Viruses as agents of genetic change

The first insight that viruses may do much more than make us sick came from studies with Rous sarcoma virus in the 1970s. The virus causes a deadly cancer in chickens. It does so by inserting a cancer-causing gene, an oncogene, right into the DNA of the host cell-a process called integration.

Scientists were curious about this oncogene and began taking a closer look at it. To their astonishment, it turned out to be a very common gene found routinely in healthy chickens. Apparently the sarcoma virus was snatching the gene from the host cell and carrying it off to other cells in other chickens. During the abduction, close association with viral genes altered the normal chicken gene, turning it into a cancer inducer.

Stranger still, this kidnapped gene was incredibly ubiquitous, appearing in a wide range of vertebrates, including fish, mice, cows, and even humans. Perhaps this ubiquity was not coincidental. Perhaps viruses were disseminating the gene not only to other chickens but throughout the animal kingdom. Taken to its logical conclusion, viruses could be agents of dispersal for a whole host of genes. This would rank the virus right up there with sexual reproduction as a major force in bringing about genetic variability and ultimately the evolution of species.

In 1977 a scientific discovery was made that truly stunned the genetics community. In that year scientists stumbled on introns in the genes of chickens and rabbits. An intron is a stretch of meaningless DNA-DNA that codes for no proteins and has, as yet, no discernible function. Not surprisingly, scientists refer to it as junk DNA.

Further investigations have turned up introns in many other animals, including humans. In fact the more highly evolved an organism, the more meaningless DNA it seems to possess. Amazingly, more than 95 percent of our own DNA is worthless stretches of introns.

Could this junk DNA be leftover bits and pieces of ancient viral infections, a fossil legacy of our evolutionary past? Yes, indeed, it could. Since the discovery of junk DNA, studies by molecular biologists have demonstrated that certain stretches of human DNA look remarkably like the genomes of certain viruses. And some viral DNA found in humans is identical to bits found in our closest nonhuman relatives, the chimpanzees. Researchers hypothesize that ten million or one hundred million years ago this virus slipped into the DNA of a primate that would later evolve into chimpanzees and humans. It might even have been a virus that at the time caused a deadly epidemic-a prehistoric AIDS. Now it is merely excess baggage in the descendant cells of those that survived its terror, a silent reminder of some ancient viral infection.

Marburg

In August 1967 three factory workers for the vaccine-producing company Behring Works developed muscle aches and mild fevers. Possibly some sort of flu, doctors first thought. As the days dragged on, however, it became apparent that this was no flu. The workers became nauseated and began to vomit. Diarrhea set in. At the same time their eyes became severely bloodshot and they developed a painful red rash-the result of blood clotting in the thousands of capillaries just under the skin. Their throats became so raw that they could not swallow and had to be fed intravenously. But the virus was just warming up. Within ten days of the onset of symptoms they began vomiting and defecating blood.

Marburg, Ebola, and several other devastating viral infections are termed hemorrhagic fevers because sufferers start bleeding profusely in the latter stages of the disease. With the body's clotting factors exhausted, blood pours out of every orifice, taking sloughed-off body tissues with it. Blood and Marburg viruses gush in all directions. If the virus lands on another human, the horrid cycle of infection will begin again.

In total, thirty-one Europeans were infected with Marburg. Then Marburg vanished just as suddenly as it had appeared. In the aftermath of the onslaught scientists worked frantically to answer the most basic questions. What kind of virus was it? Where had it come from? To answer the second question first, it was discovered early on that all of the people who originally became ill were handling monkeys or monkey tissues. Furthermore, all of the monkeys were African green monkeys imported from Uganda in three separate shipments. Wild monkeys are routinely imported from Africa and Asia by research facilities throughout the world. Manufacturers of polio vaccine must use monkeys to develop their product since poliovirus will grow only in monkey kidney cells. Each year about sixteen thousand of them make it to the United States. Obviously something was wrong with the batch that went to Germany in 1967.

Not surprisingly, many had died of massive hemorrhages. Whatever was killing the monkeys back in their jungle homes had been imported to Europe and was jumping to a new species - humans. The virus was emerging. When a virus jumps to a new animal species it, as a rule, is uncommonly deadly. The new host has never been exposed to the virus, and by the time its immune system wakes up it is too late. For this reason scientists did not believe the green monkey to be Marburg's natural host. The virus was even more devastating to these primates than to humans, killing with a ferocity approaching 100 percent. Most likely there was a reservoir of Marburg viruses hiding in some other, as yet unknown, rain forest animal.

Over the next few years the World Health Organization (WHO) as well as the United States and Europe sent teams of scientists to Kenya and Uganda to scour the countryside in search of that animal. They caught and tested tens of thousands of monkeys, apes, rodents, mosquitoes, ticks, bats, and cats. Unfortunately, the animal in which the Marburg virus resided naturally and harmlessly proved very elusive, and no reservoir for the virus was ever found.

Electron micrographs of blood and tissue samples from Marburg victims did reveal that, wherever its hiding place, the virus was unlike any other. Whereas most viruses are spherical or a similar shape, Marburg looked like a short piece of yarn. When it "amplified" itself within a cell, producing thousands of viral copies, it resembled a bowl of entangled spaghetti. Hence the agent of Marburg hemorrhagic fever was dubbed a filovirus, filo being Latin for "thread". No other virus looked like Marburg.

Since the 1967 German outbreak the virus has struck again only twice, in 1976 and again in 1990. Four people in all were infected, all Europeans, and all contracted the disease during travels in Africa. One person died. Today the virus sits, freeze-dried, in just a few laboratories throughout the world.

The scientific community is not actively engaged in studying Marburg because it is so "hot." A hot virus is one that spreads easily, kills quickly, with a high mortality rate, and has no cure or preventive vaccine. The hottest viruses, of which Marburg is a charter member, require very special handling. Doctors must wear cumbersome "spacesuits" with independent air supplies to keep all parts of their bodies from direct contact with the pathogen. Additional rubber gloves are worn beneath the suit and sometimes even over the suit, as further protection against accidental cuts or punctures while working with scalpels and hypodermic needles. Animals and tissue specimens are often handled in sealed, airtight glass and steel boxes with permanently attached gloves.

Marburg killed about 25 percent of the people it infected, making it an extremely lethal virus. Many of humanity's worst plagues did not have mortality figures that high. Yellow fever, for example, a deadly viral disease transmitted by the bite of a female mosquito (male mosquitoes do not bite), usually killed only one in ten. Yet Marburg was not the worst of the hemorrhagic fevers to come out of the jungles of Africa. Nine years after the emergence of Marburg, in the central African nations of Sudan and Zaire, an even deadlier virus struck. It came to be known as Ebola.

Ebola

Ebola first introduced itself to humanity in the southern Sudan in July 1976. A quiet man became ill and died an agonizing and bloody death. From him the virus radiated outward, infecting friends, mistresses, and family members. Before the outbreak ran its course, 284 people became infected and 150 died, a death rate of just over 50 percent.

Then, in September of that year, Ebola struck again, this time in Zaire. Ebola Zaire turned out to be a slightly different strain from Ebola Sudan; it was a super Ebola that killed a staggering 90 percent of its victims. In several months late in 1976 it swept through fifty villages, killing 325 of the 358 people it infected.

The Ebola virus can attack and amplify itself in virtually any body tissue except bone and perhaps skeletal muscle. First come the searing headache, fever, and muscle pain. Then the bleeding starts. When internal hemorrhaging first occurs, the body's clotting factors are called into play. Organs such as the liver and spleen are transformed into hardened, desiccated masses of coagulated blood and tissue. The kidneys, filtering organs for the body, become so clogged with clumped blood that they cease to function. A tremendous burden is placed on the heart as it attempts to pump this congealing mess through thousands of miles of blood vessels. When all the clotting factor available has been depleted, uncontrolled bleeding commences. The virus is now running amok, amplifying itself by the billions as it destroys the body. Capillaries deteriorate, and blood flows into the lungs, stomach, and intestine. The skin may balloon as blood leaks into the underlying tissue. Victims weep blood. Ebola is transforming its host's innards into viral soup. Mercifully death soon follows, often from shock (due to blood loss and lowered blood pressure), heart failure, or lung congestion.

The Ebola viruses proved to be close cousins of Marburg. All were filoviruses that, under the probing beam of the electron microscope, appeared nearly identical. All had similar genetic material-a single strand of RNA. And like Marburg, no natural host was ever found for either strain of Ebola.

Even more significant, the filoviruses are apparently not as contagious as originally feared. Although Marburg and Ebola can infect and kill monkeys that inhale the virus, airborne transmission is not easily accomplished. And transmission from human to human through casual contact is even less likely. During the outbreaks of 1976, the spread of Ebola was not the result of people greeting one another in the street or frequenting the same restaurants. Ironically, it was the hospitals that were largely responsible for the mini-epidemics. Medical supplies were almost nonexistent. When patients came in for antimalarial medication or treatment for various and sundry ailments, the same needles were used over and over again, with little more than a quick rinse between injections.

Lassa

Lassa fever is a hemorrhagic disease endemic to western Africa, where it is responsible for about five thousand deaths a year. It is a brutal killer in the fashion of Marburg or Ebola. Science first became aware of Lassa in 1969, when it struck down American nurses in a church-run hospital in Nigeria. The symptoms of the disease were scarily reminiscent of Marburg, which had been discovered two years earlier. Under the electron microscope, however, Lassa virus was spherical, looking nothing like a filovirus. And when the new pathogen was mixed with Marburg antibodies, no reaction occurred. Lassa was stimulating the body to produce antibodies different from Marburg. They were entirely different viruses.

Not only did Lassa not react with Marburg antibodies; it did not react with any known antibodies. This could mean only one thing: the medical establishment had yet another hot virus on its hands. Once again the hunt was on for the source, or natural host, of the virus. A gut feeling told researchers that it was not an arbovirus. For one thing, all cases appeared to have occurred indoors. For another, the disease attacked primarily adults. As a rule, insect-carried diseases show a preference for children because they play in the wet breeding grounds of mosquitoes, mites, ticks, etc. The hunch proved correct. More than 640 animals, mostly small mammals such as mice, rats, and bats, were captured in and around the affected villages. Blood was collected. Lungs, hearts, spleens, and kidneys were surgically removed. All specimens were placed in liquid nitrogen and shipped overseas to the Special Pathogens Branch for testing. When the results came in, one animal turned up positive for Lassa virus-Mastomys matalensis, a common brown rat.

The Black Death, and typhus were both monstrous killers, stealthily introduced into human communities by the rat. But these diseases were actually transmitted to humans by fleas or lice living on and biting the diseased rodents. Was this the mode of transmission for Lassa, or did the rats themselves bite human victims? Lassa sufferers did not recall being bitten. But one researcher was urinated on by an angry rat as he held it. Two weeks later the scientist died of Lassa fever. As it turned out, the urine of infected rats was teeming with viruses. And the animals were ubiquitous throughout eight countries in western Africa, finding human dwellings particularly to their liking. They urinated on food, floors, bedding, and even people as they slept. The tiniest break in the skin would afford the virus a portal into a human host.

Dengue

Dengue hemorrhagic fever is the more severe form of a related disease called simply dengue fever. Both cause excruciating bone and joint pain from which the disease derives its common name, break bone fever. In the hemorrhagic dengue, however, internal bleeding and subsequent shock lead to death 15 percent of the time.

Like its close cousin, yellow fever, dengue is an arbovirus. Dengue is spread by the bite of an arthropod. Both diseases, in fact, are transmitted by the same vector, a mosquito called A. aegypti. Mosquitoes are among the most efficient transmitters of disease. In Zaire it was reused hypodermic needles that spread Ebola so rapidly throughout hospitals. Well, a female mosquito (remember that only females bite) is a flying hypodermic needle. The needle, a long proboscis, is used to secure a blood meal. But before sticking her victim the mosquito goes through an odd ritual. She primes the targeted patch of skin by first spitting on it. The saliva, in addition to containing an anticoagulant that keeps the blood flowing, may be loaded with virus particles. Then A.aegypti inserts her proboscis and gorges herself on a huge quantity of blood, ballooning to four times her original weight. At this point the mosquito becomes more than a hypodermic syringe. If the blood she ingests happens to contain dengue viruses, they will replicate themselves in her salivary glands. When she again spits and bites, woe to the hapless victim.