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Overview of Viral Infections


Laura D Kramer

, PhD, Wadsworth Center, New York State Department of Health

Full review/revision Jun 2021 | Modified Sep 2022
Topic Resources

A virus is composed of nucleic acid, either DNA or RNA Genes Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are structures within cells... read more Genes , surrounded by a protein coat. It requires a living cell in which to multiply. A viral infection can lead to a spectrum of symptoms from asymptomatic (no overt symptoms) to severe disease.

  • People may get viruses by swallowing or inhaling them, by being bitten by insects, or through sexual contact.

  • Most commonly, viral infections involve the nose, throat, and upper airways, or systems such as the nervous, gastrointestinal, and reproductive systems.

  • Doctors may base the diagnosis on symptoms, blood tests and cultures, or examination of infected tissues.

  • Antiviral drugs may interfere with the reproduction of viruses or strengthen the immune response to the viral infection.

A virus is a small infectious organism—much smaller than a fungus or bacterium—that must invade a living cell to reproduce (replicate). The virus attaches to a cell (called the host cell), enters the cell, and releases its DNA or RNA Genes Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are structures within cells... read more Genes inside the cell. The virus’s DNA or RNA is the genetic material containing the information needed to make copies of (replicate) the virus. The virus’s genetic material takes control of the cell and forces it to replicate the virus. The infected cell usually dies because the virus keeps it from performing its normal functions. When it dies, the cell releases new viruses, which go on to infect other cells.

Some viruses do not kill the cells they infect but instead alter the cell's functions. Sometimes the infected cell loses control over normal cell division and becomes cancerous.

Did You Know...

  • A virus takes control of the cell it infects and forces it to make more viruses.

Viruses usually infect one particular type of cell. For example, common cold viruses Common Cold The common cold is a viral infection of the lining of the nose, sinuses, and throat. Many different viruses cause colds. Usually, colds are spread when a person's hands come in contact with... read more infect only cells of the upper respiratory tract. Additionally, most viruses infect only a few species of plants or animals. Some infect only people.

Types of viral infections

Probably the most common viral infections are

  • Respiratory infections: Infections of the nose, throat, upper airways, and lungs

Respiratory infections are more likely to cause severe symptoms in infants, older people, and people with a lung or heart disorder.

Other viruses infect other specific parts of the body:

Spread of viruses

Viruses are spread (transmitted) in various ways. They may be

Many viruses that were once present in only a few parts of the world are now spreading. These viruses include chikungunya virus, Crimean-Congo hemorrhagic fever virus, Japanese encephalitis virus, Rift Valley Fever virus, West Nile virus, Ross River virus, Zika virus Zika Virus Infection Zika virus infection is a mosquito-borne viral infection that typically causes no symptoms but can cause fever, rash, joint pain, or infection of the membrane that covers the white of the eye... read more Zika Virus Infection , and louping ill virus. These viruses are spreading partly because climate change has resulted in more areas where the mosquitoes that spread the viruses can live. Also, travelers may be infected, then return home and be bitten by a mosquito, which spreads the virus to other people.

Defenses against viruses

The body has a number of defenses against viruses:

  • Physical barriers, such as the skin, which discourage easy entry

  • The body's immune defenses, which attack the virus

When a virus enters the body, it triggers the body's immune defenses. These defenses begin with white blood cells White blood cells , such as lymphocytes Lymphocytes One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more Lymphocytes and monocytes, which learn to attack and destroy the virus or the cells the virus has infected. If the body survives the virus attack, some of the white blood cells remember the invader and are able to respond more quickly and effectively to a subsequent infection by the same virus. This response is called immunity. Immunity can also be produced by getting a vaccine Overview of Immunization Immunization (vaccination) helps the body defend itself against diseases caused by certain bacteria or viruses. Immunity (the ability of the body to defend itself against diseases caused by... read more .

Viruses and cancer

Only a few viruses are known to cause cancer, but there may be others.


Diagnosis of Viral Infections

  • A doctor's evaluation

  • For infections that occur in epidemics, the presence of other similar cases

  • For some infections, blood tests and cultures

For other infections, blood tests and cultures (growing microorganisms in the laboratory from samples of blood, body fluid, or other material taken from an infected area) may be done. Polymerase chain reaction (PCR) techniques may be used to make many copies of the viral genetic material. PCR techniques make it easier for doctors to rapidly and accurately identify the virus. Blood may also be tested for antigens, which are proteins on or in viruses that trigger the body's defense. Blood may also be tested for antibodies Antibodies One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more Antibodies to viruses. (Antibodies are proteins produced by the immune system to help defend the body against a particular attack.) Tests are usually done quickly, especially when the infection is a serious threat to public health or when symptoms are severe.

A sample of blood or other tissues is sometimes examined with an electron microscope, which provides high magnification with clear resolution.

Prevention of Viral Infections

Prevention of viral infections may include

Vaccines and immune globulins help the body better defend itself against diseases caused by certain viruses (or bacteria). The process of strengthening the body's defenses is called immunization Overview of Immunization Immunization (vaccination) helps the body defend itself against diseases caused by certain bacteria or viruses. Immunity (the ability of the body to defend itself against diseases caused by... read more .

General measures

People can help prevent many viral infections by commonsense measures to protect themselves and others (personal protective measures). These measures vary depending on the how the virus is spread. Measures include the following:


Viral vaccines in general use include the following:

Multiple COVID-19 vaccines COVID-19 Vaccine Coronavirus disease 2019 (COVID-19) vaccines provide protection against COVID-19. COVID-19 is the disease caused by infection with the SARS-CoV-2 virus. There are multiple COVID-19 vaccines... read more are currently in use worldwide. In the US, three vaccines have emergency use authorization (EUA) from the US Food and Drug Administration (FDA), but full approval is still pending.

An Ebola vaccine Ebola Vaccine rVSV-ZEBOV is the only vaccine approved for use in the United States by the U.S. Food and Drug Administration (FDA) for prevention of Ebola virus disease caused by Zaire ebolavirus species... read more in use in West Africa on a limited scale during outbreaks since 2016 was approved by the US Food and Drug Administration in December 2019 for use in people 18 years of age and older.

Viral diseases can be eradicated by good vaccines. Smallpox Smallpox Smallpox is a highly contagious, very deadly disease caused by the variola virus. The disease is now considered eliminated. There have been no cases of smallpox since 1977. People can acquire... read more Smallpox was eradicated in 1978. Polio Polio Polio is a highly contagious, sometimes fatal enterovirus infection that affects nerves and can cause permanent muscle weakness, paralysis, and other symptoms. Polio is caused by a virus and... read more has been eradicated from all but a few countries where logistics and religious sentiment continue to interfere with vaccination. Measles Measles Measles is a highly contagious viral infection that causes various cold-like symptoms and a characteristic rash. Measles is caused by a virus. Symptoms include fever, runny nose, hacking cough... read more Measles has been almost eradicated from some parts of the world, such the Americas. However, because measles is highly contagious and vaccination coverage is incomplete even in regions where it is considered eradicated, it is not likely to be completely eliminated soon.

Immune globulins

Immunoglobulins can be collected from the blood of the following:

  • People who are generally healthy (these immunoglobulins are called pooled human immunoglobulin)

  • People who have many antibodies that defend against a specific infectious organism, often because they have been infected with that organism (these immunoglobulins are called hyperimmune globulin)

Immune globulins are given by injection into a muscle or into a vein. The immunity provided by immune globulins lasts for only a few days or weeks, until the body eliminates the injected antibodies.

Sometimes, such as when people are exposed to rabies or hepatitis B, they are given both immune globulin and a vaccine to help prevent infection from developing or reduce the severity of infection.

Treatment of Viral Infections

  • Treatment of symptoms

  • Sometimes antiviral drugs

Treatment of symptoms

There are no specific treatments for many viruses. However, many things can help relieve certain symptoms, such as the following:

Not everyone who has these symptoms needs treatment. If symptoms are mild, it may be better to wait for them to go away on their own. Some treatments may not be appropriate for infants and young children.

Antiviral drugs

Drugs that combat viral infections are called antiviral drugs. There are no effective antiviral drugs for many viral infections. However, there are several drugs for influenza Treatment Influenza (flu) is a viral infection of the lungs and airways with one of the influenza viruses. It causes a fever, runny nose, sore throat, cough, headache, muscle aches (myalgias), and a general... read more , many drugs for infection by one or more herpesviruses (see table Some Antiviral Drugs for Herpesvirus Infections Some Antiviral Drugs for Herpesvirus Infections Some Antiviral Drugs for Herpesvirus Infections ), and many new antiviral drugs for treatment of HIV Drug Treatment of Human Immunodeficiency Virus (HIV) Infection Antiretroviral drugs used to treat human immunodeficiency virus (HIV) infection aim to do the following: Reduce the amount of HIV RNA (viral load) in the blood to an undetectable amount Restore... read more (see table Drugs for HIV Infection ) , hepatitis C Treatment Chronic hepatitis is inflammation of the liver that lasts at least 6 months. Common causes include hepatitis B and C viruses and certain drugs. Most people have no symptoms, but some have vague... read more , hepatitis B Hepatitis B, Chronic Chronic hepatitis B is inflammation of the liver that is caused by the hepatitis B virus and that has lasted more than 6 months. Most people with chronic hepatitis B have no symptoms, but some... read more , and Ebola Ebola Virus and Marburg Virus Infections Marburg and Ebola virus infections cause bleeding and organ malfunction. These infections often result in death. Marburg and Ebola infections are spread through handling live or dead infected... read more .

Many antiviral drugs work by interfering with replication of viruses. Most drugs used to treat HIV infection work this way. Because viruses are tiny and replicate inside cells using the cells' own metabolic functions, there are only a limited number of metabolic functions that antiviral drugs can target. In contrast, bacteria are relatively large organisms, commonly reproduce by themselves outside of cells, and have many metabolic functions that antibacterial drugs (antibiotics) can target. Therefore, antiviral drugs are much more difficult to develop than antibiotics. Also, unlike antibiotics, which are usually effective against many different species of bacteria, most antiviral drugs are usually effective against only one (or a very few) viruses.

Antiviral Drugs

Antiviral drugs can be toxic to human cells. Also, viruses can develop resistance to antiviral drugs.

Most antiviral drugs can be given by mouth. Some can also be given by injection into a vein (intravenously) or muscle (intramuscularly). Some are applied as ointments, creams, or eye drops or are inhaled as a powder.

Antibiotics are not effective against viral infections, but if a person has a bacterial infection in addition to a viral infection, an antibiotic is often necessary.

Interferon drugs are replicas of naturally occurring substances that slow or stop viral replication. These drugs are used to treat certain viral infections such as

Interferons may have side effects, such as fever, chills, weakness, and muscle aches. These effects typically start 7 to 12 hours after the first injection and last up to 12 hours.

Antibodies from the blood of a person who has recovered from the viral infection (convalescent serum) and antibodies that are produced in a laboratory from living cells that have been altered to produce the desired antibodies (monoclonal antibodies Monoclonal Antibodies Immunotherapy is the use of drugs that mimic or modify components of the immune system (such as tumor antigens and immune checkpoints—see also Overview of the Immune System) to fight disease... read more ) are used to treat some viral infections including

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