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Summary Article: Viral Infections
From Encyclopedia of Global Health

There are more than 400 different viruses that are capable of infecting humans. Other viruses such as bacteriophages (viruses that infect bacteria), plant viruses and animal viruses also exist. There are many means by which a viral transmission can occur. For instance, they can be airborne viruses, arthropod-borne viruses, person-to-person transmitted viruses, food- and waterborne viruses, sexually transmitted viruses, and prions.

Infection occurs when a virus takes over the cell machinery and makes many copies of it. While the virus replicates, host cells are either destroyed or are reduced in their normal functional activity. The progeny that is released then continues the cycle by affecting other cells and multiplying to form more copies of the virus. It ultimately ends up destroying the host cells completely. A virus particle or virion can be made up of a single- or double-stranded DNA or RNA. Virions are obligate intracellular pathogens that absolutely require a host cell to reproduce. Thus, viruses work in a simple way and yet give rise to a range of diseases by means of genetic variation. This type of simplicity of viruses that leads to intense pathological features, clinical consequences and death is what makes virology (study of viruses) important.

A virus under magnification: A viral infection may be systemic or localized, depending upon the type of virus that infects the body.

Infection caused by virus can affect all types of body tissue. Symptoms for viral infections include headache, high fever, weakness, muscle pain, nausea, vomiting, diarrhea, and abdominal pain. Although there is no accurate treatment available for viral infections, physicians usually treat them with antibiotics. Antiviral drugs against some viruses are also available. A viral infection may be systemic (spread throughout the body) or localized (infect only at one location) depending upon the type of virus that infects.


Epstein-Barr virus is a chief cause of infectious mononucleosis or mono. A person usually gets infected by transmission of virus through oral contact. Kissing an infected person is a major mode of transmission. The virus can then spread to other organs of the body. Although most cases of mono occur due to Epstein-Barr virus infection, a virus known as cytomegalovirus is also capable of resulting into this type of illness. Stress and anxiety are also linked to the cause of infection. Young adults are most susceptible.


Shingles is a condition that affects adults due to reactivation of an infection caused by chicken pox virus. The varicella zoster virus, which causes chicken pox infection in early years, causes shingles in adults by remaining in a dormant state. The virus stays inactive in the brain tissue, spinal cord, or nerve cells for years. Reactivation from this state takes place in reaction to external factors that stimulate the virus to switch back to an active form. Stress, cancer, hormonal imbalance, and weakened immune system are examples of situations under which the virus might activate. These types of stimulations can cause many reoccurrences of the shingles infection. Shingles cannot be transmitted from person to person. On the contrary, an individual with no previous infection of varicella zoster virus can develop chicken pox if transmission of virus from a shingles-infected person takes place. Rashes or blisters with severe pain are an initial symptom. This pain may continue even after complete recovery from blisters resulting in a condition known as postherpetic neuralgia.

All body parts are at equal risk of being affected. If the infection occurs in facial region or near the eyes of a patient, then it may lead to temporary paralysis of face and/or low vision. In addition, infection to auditory nerves may result in hearing problems.


Dengue virus spreads infection to humans via mosquito bites. Aedes are female mosquitoes (the same mosquitoes that transmit malaria) that are capable of infecting with dengue. This virus thrives around water pools and in unsanitary conditions. Cleanliness to eradicate the Aedes mosquitoes is an important measure to be taken when an area is threatened by dengue. Central and south America, southeast Asia, China, India, Caribbean, Africa, and Middle East are areas most hit by this arthropod-borne infection.


Rabies virus spreads to humans via an intermediate host that is infected. For example, when an infected animal bites or scratches a person, it can transfer the virus. When the infection is passed on, the rabies virus finds its way into the nearest nerve ending that will lead it to the brain leading to a life-threatening illness. Household pets can be vaccinated against rabies virus to prevent them from getting infected. However, wild animals such as bats, foxes, coyotes, and raccoons seldom transmit the infection to a human host.


Influenza viruses transmit from person to person and are causative agents of a highly contagious, often serious acute respiratory illness. The virus is passed by a respiratory droplet transmission. For example, coughing or sneezing by an infected individual can release the influenza virus in the air. The virus then can infect other available hosts. It takes about one to four days for influenza virus to incubate in the host before it starts shedding and becomes infectious.

Influenza in immunocompromised patients can be critical. In addition, patients with severe influenza infection may suffer from secondary acute infections caused by bacteria and other pathogens. Symptoms may include headache, fever, cough, sore throat, and malaise.

Influenza viruses are of different subtypes. For example, the avian influenza virus responsible for causing bird flu is an influenza A virus, also known as the H5N1 virus. H5N1 virus does not spread from human to human. Study of influenza virus is important as it gives rise to pandemics from time to time. This happens as the virus forms a new strain by undergoing a genetic shift. A new strain of infectious virus originates and affects the human population when an influenza virus from bird and a human influenza virus combine to form a new strain in an intermittent host reservoir, such as a pig. This new virus from pigs is then capable of infecting humans. Although the avian flu virus is not directly harmful to humans, H5N1 cases have occurred due to direct handling of birds.

Influenza vaccines can prevent a person from getting an infection. Most cases of influenza are treated by using antiviral drugs amantidine and rimatadine. H5N1 virus in Asia is now resistant to these two antiviral and is provided by using other antiviral medications such as oseltamavir and zanamavir. Vaccine for H5N1 is yet to be discovered and so there is no prevention regimen available at present. Avian flu in Asia has led to severe illness and death similar to that of 1918 pandemic of Spanish flu. The most susceptible age group then was young adults and elderly patients older than 65 years. It was a devastating disease that killed more people than World War I.


Different viruses give rise to many types of diseases. Airborne viral diseases such as chicken pox, shingles, and influenza can occur directly or indirectly through the respiratory system. Arthropod-borne viral diseases, which include West Nile virus and yellow fever, can be transmitted to humans from arthropod vectors that infect other human or animal hosts. Viral diseases such as AIDS, genital herpes, hepatitis, rabies, and infectious mononucleosis are transmitted from person-to-person contact. Sexually transmitted diseases (STDs) usually spread through oral or fecal sexual contact. It is transmitted via the medium of blood or sputum.

Food-and waterborne viruses enter the body via oral route and usually infect the intestinal system. They are shed in the feces of an infected person and can then spread to other individuals who come in contact. Acute viral gastroenteritis caused by rotavirus, hepatitis A, and poliomyelitis are examples of such viruses.

Some viruses may spread by autoinoculation such as direct or indirect contact, scratching, or through an open wound. Common skin warts caused by human papillomaviruses fall under this category. A prion is a small protein particle that remains dormant for a long time before the onset of symptoms. Kuru, fatal familial insomnia and progressive multifocal leukoencephalopathy are few examples. Because most prions infect the central nervous system, infections caused by them are chronic and life threatening.

  • AIDS and Infections; Dengue; Genital Herpes

  • Guillain-Barre Syndrome; Herpes Simplex; Immune System and Disorders; Immunosuppression; Influenza; Measles; Meningitis; Mumps; Rabies; Smallpox; Urinary Tract Infections; Virology.

  • American Medical Association Family Medical Guide, 4th ed. (Wiley, 2004).
  • Department of Health and Human Services: Centers for Disease Control (CDC) and Prevention, “Influenza Flu: Clinical Description and Diagnosis,” (cited January 2007).
  • Martins S. Lipsky, American Medical Association: Concise Medical Encyclopedia (Random House Reference, 2007).
  • MedicineNet.Com: We Bring Doctors Knowledge to You, “Definition of Viral Infection,” (cited January 2007).
  • Prescott Harley Klein, et al., Microbiology, 5th ed. (McGraw-Hill, 2002).
  • Stanford University, “The Influenza Pandemic of 1918,” (cited January 2007).
  • Jinal Mehta
    University of Missouri-Kansas City
    Copyright © 2008 by SAGE Publications, Inc.

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