Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Overview of Viral Infections

By

Laura D Kramer

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

Reviewed/Revised Mar 2023
VIEW PROFESSIONAL VERSION
GET THE QUICK FACTS
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 or the code for functional ribonucleic... 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, through sexual contact, or congenitally (passed by a pregnant person to the fetus).

  • 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 or the code for functional ribonucleic... read more Genes inside the cell. The virus’s DNA or RNA is the genetic material containing the information needed to replicate the virus. The virus’s genetic material takes control of the host cell and forces it to replicate the virus. The infected cell usually dies because the virus keeps it from performing its normal functions. When the infected host cell dies, it 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 causes 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

Upper respiratory infections (infections of the nose, throat, upper airways, and lungs) are likely the most common viral infections.

Upper respiratory infections include sore throat Sore Throat Sore throat is pain in the back of the throat. A sore throat can be very painful and is usually worsened by swallowing. Many people with a sore throat refuse to eat or drink. Sometimes pain... read more Sore Throat , sinusitis Sinusitis Sinusitis is inflammation of the sinuses, most commonly caused by a viral or bacterial infection or by an allergy. Some of the most common symptoms of sinusitis are pain, tenderness, nasal congestion... read more , and the common cold 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 . Other viral respiratory infections include influenza Influenza (Flu) 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 , pneumonia Community-Acquired Pneumonia Community-acquired pneumonia is lung infection that develops in people who are not patients in a hospital, usually in people with a normal (competent) immune system, or in those who are immunocompromised... read more , and coronaviruses Coronaviruses and Acute Respiratory Syndromes (MERS and SARS) Coronaviruses are a large family of viruses that cause respiratory illness ranging in severity from the common cold to fatal pneumonia. There are many different coronaviruses. Most of them cause... read more , including SARS-CoV-2 COVID-19 COVID-19 is an acute respiratory illness that can be severe and is caused by the coronavirus named SARS-CoV-2. Symptoms of COVID-19 vary significantly. Two types of tests can be used to diagnose... read more (the virus that causes COVID-19).

Respiratory infections are more likely to cause severe symptoms in infants, older people, and people with a lung or heart disorder. Respiratory viruses are typically spread from person to person by contact with infected respiratory droplets.

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 West Nile Virus Infection West Nile virus infection is a viral disease spread primarily from mosquitoes to people. Most people have mild or no symptoms, but some people develop a severe infection that affects the central... read more , 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 or ticks 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.

Table

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.

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

Medications that combat viral infections are called antiviral drugs. Many viral infections do not have effective antiviral drugs available to treat them. 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 ), and many antiviral drugs for treatment of HIV Antiretroviral Treatment of Human Immunodeficiency Virus (HIV) Infection Antiretroviral medications 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... read more , 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 COVID-19 COVID-19 COVID-19 is an acute respiratory illness that can be severe and is caused by the coronavirus named SARS-CoV-2. Symptoms of COVID-19 vary significantly. Two types of tests can be used to diagnose... read more , which is caused by SAR-CoV-2.

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
VIDEO

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.

Interferons 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.

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 routine measures to protect themselves and others (personal protective measures). These measures vary depending on the how the virus is spread. Measures include the following:

Vaccines

Viral vaccines in general use include the following:

Viral diseases can be eradicated by effective 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. Extensive vaccination has almost eradicated 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 worldwide, but cases still occur in areas with incomplete immunization, such as sub-Saharan Africa and southern Asia. 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 as 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.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Anti-Diarrheal, Imodium A-D, Imodium A-D EZ Chews , K-Pek II
7T Gummy ES, Acephen, Aceta, Actamin, Adult Pain Relief, Anacin Aspirin Free, Aphen, Apra, Children's Acetaminophen, Children's Pain & Fever , Children's Pain Relief, Comtrex Sore Throat Relief, ED-APAP, ElixSure Fever/Pain, Feverall, Genapap, Genebs, Goody's Back & Body Pain, Infantaire, Infants' Acetaminophen, LIQUID PAIN RELIEF, Little Fevers, Little Remedies Infant Fever + Pain Reliever, Mapap, Mapap Arthritis Pain, Mapap Infants, Mapap Junior, M-PAP, Nortemp, Ofirmev, Pain & Fever , Pain and Fever , PAIN RELIEF , PAIN RELIEF Extra Strength, Panadol, PediaCare Children's Fever Reducer/Pain Reliever, PediaCare Children's Smooth Metls Fever Reducer/Pain Reliever, PediaCare Infant's Fever Reducer/Pain Reliever, Pediaphen, PHARBETOL, Plus PHARMA, Q-Pap, Q-Pap Extra Strength, Silapap, Triaminic Fever Reducer and Pain Reliever, Triaminic Infant Fever Reducer and Pain Reliever, Tylenol, Tylenol 8 Hour, Tylenol 8 Hour Arthritis Pain, Tylenol 8 Hour Muscle Aches & Pain, Tylenol Arthritis Pain, Tylenol Children's, Tylenol Children's Pain+Fever, Tylenol CrushableTablet, Tylenol Extra Strength, Tylenol Infants', Tylenol Infants Pain + Fever, Tylenol Junior Strength, Tylenol Pain + Fever, Tylenol Regular Strength, Tylenol Sore Throat, XS No Aspirin, XS Pain Reliever
Zofran, Zofran in Dextrose, Zofran ODT, Zofran Solution, Zuplenz
4-Way Nasal, Ah-Chew D, AK-Dilate, Anu-Med, Biorphen, Formulation R , Foster & Thrive Nasal Decongestion, Gilchew IR, Hemorrhoidal , IMMPHENTIV, Little Remedies for Noses, Lusonal, Mydfrin, Nasop, Nasop 12, Neofrin, Neo-Synephr, Neo-Synephrine, Neo-Synephrine Cold + Allergy, Neo-Synephrine Extra Strength, Neo-Synephrine Mild, Neo-Synephrine Non-Drowsy Cold + Allergy, Ocu-Phrin, PediaCare Children's Decongestant, PediaCare Decongestant, PediaCare Infants' Decongestant, Sinex Nasal, Sudafed PE, Sudafed PE Children's Nasal Decongestant , Sudafed PE Congestion, Sudafed PE Sinus Congestion, Sudogest PE, Vazculep
Advocate Pain Relief Stick, Americaine, Anbesol, Anbesol Baby , Anbesol Jr , Banadyne-3, Benzodent, Benz-O-Sthetic, Boil-Ease, Cepacol Sensations, Chloraseptic, Comfort Caine , Dry Socket Remedy, Freez Eez, HURRICAINE, HURRICAINE ONE, Little Remedies for Teethers, Monistat Care, Orabase, OraCoat CankerMelts, Orajel, Orajel Baby, Orajel Denture Plus, Orajel Maximum Strength, Orajel P.M., Orajel Protective, Orajel Severe Pain, Orajel Swabs, Orajel Ultra, Oral Pain Relief , Oticaine , Otocain, Outgro, Pinnacaine, Pro-Caine, RE Benzotic, Topex, Topicale Xtra, Zilactin-B
NOTE: This is the Consumer Version. DOCTORS: VIEW PROFESSIONAL VERSION
VIEW PROFESSIONAL VERSION
quiz link

Test your knowledge

Take a Quiz!
iOS ANDROID
iOS ANDROID
iOS ANDROID
TOP