A virus is defined as an infectious biological agent that can only replicate within the cells of any living host. When a host cell is infected by a virus, it is forced to generate several thousand similar copies of the virus that originally infected it. This type of replication continues at an amazing pace.

Different from nearly all other living organisms, viruses actually do not possess cells that can reproduce. In fact, new viruses are accumulated in the host cell that has already been infected. On the other hand, unlike several simpler contagious agents, viruses have genes, which make it possible for them to mutate and evolve further. As of now, more than 5,000 virus species have been identified.

How and where exactly viruses originated is still not clear. It is possible that some species developed from plasmids (a small segment of DNA with the ability to move between the cells), while there are others that may have developed from bacteria. A virus comprises two parts - one is the genes that have been composed of either DNA or RNA, which is basically, elongated molecules carrying genetic information. Usually, the genes are protected by a protein coat. In the case of some other viruses, the genes are enveloped by fat, which encircles as well as shields them when they are not present inside the host cell. The shape of viruses may differ, ranging from the common icosahedral and helical to very complex structures. Even the size of viruses may vary. While some viruses may be as minute as 20 nanometres, there may be others that measure 300 nanometres. To give you an idea about their size, you may require placing anything between 30,000 to 750,000 viruses side by side to make them stretch to about 1 cm.

There are several ways by which viruses can spread from one host to another. In fact, it has been found that many virus species only invade particular hosts or tissues. Moreover, every virus species have different propagation methods. Usually, plant viruses move from one plant to another carried by insects or other organisms, which are called vectors. Some animal viruses, including those found in humans, usually transmit when the host comes in contact with bodily fluids already infected by these contagious organisms. For instance, viruses like influenza generally spread via the air by means of moisture droplets, which are formed when affected people sneeze or cough.

On the other hand, viruses like norovirus more often than not spread through the fecal-oral route, including food, water as well as contaminated hands. Rotavirus usually spread when people come in direct contact with children who are already infected. Another virus, the human immunodeficiency virus (HIV) is mostly transmitted by means of bodily fluids during intercourse. There are various other viruses, for instance the Dengue virus, which spread through insects that suck blood.

Infections by viruses may result in various types of diseases in animals, including humans, and plants too. However, if the immune system is potent enough, it will eliminate such disease and also provide life-long immunity to the host against the particular virus. While antibiotics do not work against viral infections, there are several anti-viral drugs, which can effectively treat severe conditions. Moreover, many vaccines have been developed, which can offer life-long immunity against infections by specific viruses.

Types of human viruses

Viruses that attack humans can cause various types of diseases. To a great extent, this depends on the type of virus as well as the tissues that are infected. Throughout their life, all humans come in contact with various types of viruses. While some viral infections can be moderate in nature, there are others that can prove to be fatal. The most common types of viruses encountered by humans across the globe include cold, flu, hepatitis and stomach viruses.

Cold viruses
There is no doubt that common cold is one of the most troublesome and omnipresent afflictions suffered by humans. As many as one billion cases of head cold are reported in the United States alone every year. Infection of the nasal passage by this virus may result in sneezing, runny nose, scratchy throat and watery eyes. Usually, this condition lasts for anything between a week and a fortnight. According to the Centers for Disease Control and Prevention (CDC), common cold or head cold can be caused by over 200 different types of virus strains known to mankind. Nevertheless, head colds are commonly attributed to rhinoviruses. Other viruses that may be responsible for this condition include coronavirus, adenovirus, echovirus, coxsackievirus, paramyxovirus, orthomyxovirus, enterovirus, and the respiratory syncytial virus.
Flu viruses
There are three different viruses that are responsible for influenza, which is commonly referred to as flu. While the influenza virus types A and B are responsible for seasonal flu infections, influenza type C virus causes a mild ailment which occurs very rarely. Season flu infections generally occur between the end of fall and the beginning of spring. The common symptoms related to flu include fever, body ache, headache, fatigue, scratchy throat, stuffy nose and dry cough. Immunization against influenza helps to protect against such viral infections, especially the influenza virus types A and B.
Stomach and intestinal viruses
Viruses that afflict our digestive system usually attack the stomach and upper intestine tissues, resulting in viral gastroenteritis. The common symptoms of viral infections of the digestive tract include diarrhea, pain and cramps in the abdominal region, vomiting and nausea. Often, rotavirus is the culprit for this condition, especially in young children and infants. The symptoms of rotavirus generally include watery diarrhea, vomiting and fever. Another common stomach virus is norovirus, which affects children as well as adults. Infection by norovirus is predominant among children and the symptoms mainly include diarrhea. In the case of adults, norovirus causes continuous vomiting. Aside from these, other digestive system viruses include astrovirus, adenovirus and sapovirus.
Hepatitis viruses
As is evident from the name, hepatitis viruses afflict our liver, resulting in inflammation. There are five different viruses that are responsible for hepatitis. These viruses are denoted by the names Hepatitis A to E. In the United States, Hepatitis viruses A, B and C are predominant. While one comes in contact with Hepatitis A virus by consuming food or water that has been contaminated by stool, the ailment caused by this virus is short-lived. On the other hand, the ailment caused by Hepatitis B can result in persistent and severe liver problems. Hepatitis B virus is found in semen and blood and it can be transmitted by sexual intercourse, mother-to-child transmission at any stage in pregnancy, childbirth and even by sharing the same injection equipment. Infection by Hepatitis C virus occurs when a person comes in contact with blood that is already infected. The most common route of transmission of this virus is via sharing the same injection equipment. Occasionally, Hepatitis C infection may also occur from mother to child. Usually, infections by Hepatitis C virus are chronic. However, the good news is that in several instances the infection may be cleared using medications.
Other human viruses
If we make a list of all the human viruses, it will take volumes. Moreover, the list will never be complete, as scientists are discovering newer human viruses every time. While several viruses like the ones discussed above are common and treatments/ immunizations too are available to deal with them, there are some uncommon ones too. The latter type of human viruses is of great concern to mankind owing to their potential deadliness. The Ebola and rabies viruses are two examples of such dangerous human viruses. Some common human viruses that are responsible for a variety of ailments, for instance the contagious agents belonging to the herpes virus family, cause genital herpes, cold sores, chickenpox, infectious mononucleosis, shingles and several other ailments. The human papillomaviruses (HPV) are responsible for the common skin warts, but in some instances they may also be a precursor to cervical cancer. There a number of newer viruses, such as SARS, HIV and MERS-CoV, which are of great concern, mainly because treatment for the diseases caused by them comes several years after the discovery of the viruses.