Viruses are packages of genes in protein coats and are usually much smaller than bacteria. Viruses are obligate parasites that must infect a host cell in order to replicate themselves. Whether viruses are alive or not is a matter of opinion; however, viruses are certainly not living cells. Virus particles (virions) do contain genetic information in the form of DNA or RNA but are incapable of growth or division by themselves. A virus may have its genome made of DNA or RNA, but only one type of nucleic acid is present in the virion of any given type of virus.
Viruses lack the machinery to generate their own energy or to synthesize protein. After invading a host cell, the virus does not grow and divide like a cell itself. The virion disassembles and the viral genes are expressed using the machinery of the host cell. In particular, viral proteins are made by the host cell ribosomes, using virus genetic information. In many cases, only the virus DNA or RNA enters the host cell and the other components are abandoned outside. After infection, virus components are manufactured by the infected cell, as directed by the virus, and are assembled into new virus particles. Usually the host cell is killed and disintegrates. Typically, several hundred viruses may be released from a single infected cell. The viruses then abandon the cell and look for another host. (Note that some viruses cause “chronic” or “persistent” infections where virus particles are made slowly and released intermittently rather than as a single burst. In this case the host cell may survive for a long time despite infection. In addition, many viruses may persist inside the host cell for a long time in a latent, non-replicating state and only change to replicative mode under certain conditions.
The most important virus costing billions of dollars from our economy are:
Astrovirus – Infection with this virus causes gastroenteritis in people, both children and adults. The infection is typically not severe but may sometimes lead to dehydration.
Chickenpox – This infection is caused by the varicella zoster virus. It is highly contagious, being spread through coughing, sneezing, or contact with secretions. The main noticeable symptom is a skin rash.
Dengue Virus – This virus is mosquito-borne and causes Dengue Fever. This illness occurs predominantly in the tropics and may be life-threatening.
Ebola – Infection with this virus can lead to hypovolemic shock due to coagulation problems caused by the virus. It is spread through body fluids, conjunctivae, and also orally.
Foot-and-mouth disease virus – This virus causes foot-and-mouth disease, a disease that spreads rapidly through certain animals but that is extremely rare in humans. Symptoms in humans can include fever, vomiting, malaise, and occasionally skin lesions.
Hepatitis A – The Hepatitis A virus causes the illness Hepatitis A. The virus is spread through feces-contaminated food and water. The illness itself is usually mild, with symptoms such as fever, nausea, abdominal discomfort, and malaise.
Hepatitis B – This virus causes the disease Hepatitis B. This disease infects the liver and can lead to liver cirrhosis and even liver cancer. The virus is transmitted through infected blood or body fluids with infected blood.
Hepatitis C – Hepatitis C infects the liver and is caused by the Hepatitis C virus. This virus is transmitted from person-to-person through contaminated blood.
Herpes – Herpes is a group of several viruses that infect humans. Diseases caused by the herpes viruses include cold sores, genital herpes, chickenpox, and shingles. Herpes viruses are spread between people by way of body fluids, through the air, and through contaminated objects and surfaces.
HIV – Human immunodeficiency virus (HIV) is the virus that causes AIDS. The virus attacks the immune system, which eventually allows other diseases to infect and proliferate in the body. HIV can be spread through blood, semen, breast milk, and vaginal fluid.
HPV Human Papilomavirus – The HPV virus is spread through sexual contact. There are hundreds of different strains of HPV, some of which cause no ill health effects. Other strains of the virus can cause genital warts and cancers of the cervix and vulva.
Influenza – Influenza is commonly known as the flu and is caused by influenza viruses. These viruses can be found in both animals and humans. The flu viruses are spread through humans by way of respiratory secretions.
Japanese Encephalitis – This is a mosquito-borne virus that produces the disease Japanese encephalitis. It can cause severe symptoms in humans but is not transmitted between them.
Measles – The measles virus is the cause of the disease measles. The virus only infects humans and is spread through respiratory secretions. Complications may occur and can include pneumonia, encephalitis, and even death.
Mumps – The mumps virus causes the disease mumps in people. The disease is transmitted from person-to-person through respiratory secretions but is generally a self-limiting disease and is not fatal in most circumstances.
Naples Virus – This is a type of Phlebovirus that has been linked to disease in humans. The virus is transmitted through infected sandflies.
Parvovirus – Parvovirus B19 is a human parvovirus that causes a disease in humans called fifth disease. This disease occurs most often in children and is characterized by a rash. Other types of parvoviruses are common in animals and cannot be transmitted from animals to humans.
Rabies – The rabies virus causes a fatal disease in animals and humans. Transmission of the virus often occurs through the saliva of animals.
Rubella – This virus causes the disease Rubella, and it also causes congenital rubella syndrome in developing fetuses and newborns. The rubella virus is only known to infect humans.
Shingles – This is a skin rash caused by the varicella zoster virus. In children, this virus causes chickenpox. Symptoms of shingles include skin pain and burning and a rash.
Smallpox – The variola virus causes smallpox. Symptoms of smallpox can include body aches, fever, vomiting, malaise, and a rash of fluid-filled bumps that cover the body.
Toscana Virus – This virus may be transmitted to humans through the bite of a sandfly. Symptoms of the illness may be mild, such as muscle aches, fever, or headache, or they may become severe, such as meningoencephalitis or meningitis.
Varicella Zoster Virus – This is the virus that causes chickenpox in children and causes shingles in adults. After a person has had a bout with chickenpox, the virus will lie dormant in the body and may become active again decades later and produce the condition shingles.
West Nile Virus – This virus can be found in both tropical and temperate areas. It is known to infect several animal species and also humans through infected mosquitoes. Infection symptoms can range from asymptomatic all the way to encephalitis-like symptoms.
Yellow Fever – This virus causes the disease yellow fever and is transmitted through the bite of a mosquito. It is found in tropical regions of South America and Africa.
Viral diseases usually cannot be cured once they have been caught. Either the victim’s body fights off the infection or it does not, although some antiviral drugs can help the host in the fight. However, viral diseases can often be prevented by immunization if a potential victim is vaccinated before catching the virus. In this case, the invading virus will be killed by the immune system, which has been put on alert by the vaccine, and the disease will be prevented.
Viruses may carry genes from one host organism to another, in a process known as transduction, and have thereby played a major role in molecular evolution. The ability of viruses to carry genes between organisms is also used by genetic engineers hoping to deliver “normal” genes into a person who has a genetic disease.