Categorizing viral infections by the organ system most commonly affected (eg, lungs, gastrointestinal tract, skin, liver, central nervous system, mucous membranes) can be clinically useful, although certain viral disorders (eg, mumps) are hard to categorize. Many specific viruses and the disorders they cause are also discussed elsewhere in THE MANUAL.
(See also Overview of Viruses Overview of Viruses Viruses are among the smallest microbes, typically ranging from 0.02 to 0.3 micrometer, although several very large viruses up to 1 micrometer in length (megavirus, pandoravirus) have recently... read more .)
Upper respiratory infections are likely the most common viral infections. Respiratory infections Overview of Viral Respiratory Infections Viral infections commonly affect the upper or lower respiratory tract. Although respiratory infections can be classified by the causative virus (eg, influenza), they are generally classified... read more are more likely to cause severe symptoms in infants, the elderly, and patients with a lung or heart disorder. Respiratory viruses are typically spread from person to person by contact with infected respiratory droplets.
Respiratory viruses include the epidemic influenza viruses Influenza Influenza is a viral respiratory infection causing fever, coryza, cough, headache, and malaise. Mortality is possible during seasonal epidemics, particularly among high-risk patients (eg, those... read more (A and B), H5N1 and H7N9 avian influenza A viruses Avian Influenza Avian influenza is caused by strains of influenza A that normally infect only wild birds and domestic poultry. Infections due to some of these strains have been detected in humans. Human-to-human... read more , parainfluenza viruses Parainfluenza Virus Infections Parainfluenza viruses include several closely related viruses that cause many respiratory illnesses varying from the common cold to an influenza-like syndrome or pneumonia; croup is the most... read more 1 through 4, adenoviruses Adenovirus Infections Infection with one of the many adenoviruses may be asymptomatic or result in specific syndromes, including mild respiratory infections, keratoconjunctivitis, gastroenteritis, cystitis, and primary... read more , respiratory syncytial virus A and B and human metapneumovirus Respiratory Syncytial Virus (RSV) and Human Metapneumovirus Infections Respiratory syncytial virus and human metapneumovirus infections cause seasonal lower respiratory tract disease, particularly in infants and young children. Disease may be asymptomatic, mild... read more , rhinoviruses Common Cold The common cold is an acute, usually afebrile, self-limited viral infection causing upper respiratory symptoms, such as rhinorrhea, cough, and sore throat. Diagnosis is clinical. Handwashing... read more , and coronaviruses Coronaviruses and Acute Respiratory Syndromes (MERS and SARS) Coronaviruses are enveloped RNA viruses that cause respiratory illnesses of varying severity from the common cold to fatal pneumonia. Numerous coronaviruses, first discovered in domestic poultry... read more .
Several coronaviruses cause a respiratory infection that can be severe. In 2002 and 2003, an outbreak of severe acute respiratory syndrome Severe Acute Respiratory Syndrome (SARS) Coronaviruses are enveloped RNA viruses that cause respiratory illnesses of varying severity from the common cold to fatal pneumonia. Numerous coronaviruses, first discovered in domestic poultry... read more (SARS) caused a number of deaths mostly in China and Hong Kong. There have been no cases of SARS reported since 2004. In 2012, a novel coronavirus Middle East respiratory syndrome coronavirus Middle East Respiratory Syndrome (MERS) Coronaviruses are enveloped RNA viruses that cause respiratory illnesses of varying severity from the common cold to fatal pneumonia. Numerous coronaviruses, first discovered in domestic poultry... read more (MERS-CoV) appeared in Saudi Arabia; it can cause severe acute respiratory illness and is often fatal (case fatality rate of 34%). In 2019, another coronavirus (SARS-CoV2) that can cause an acute, sometimes fatal respiratory illness (COVID-19 COVID-19 COVID-19 is an acute, sometimes severe, respiratory illness caused by the novel coronavirus SARS-CoV-2. Prevention is by vaccination and infection control precautions (eg, face masks, handwashing... read more ) emerged in Wuhan, China and spread in a worldwide pandemic that has killed millions of people (1 Reference Categorizing viral infections by the organ system most commonly affected (eg, lungs, gastrointestinal tract, skin, liver, central nervous system, mucous membranes) can be clinically useful,... read more ).
Gastroenteritis Overview of Gastroenteritis Gastroenteritis is inflammation of the lining of the stomach and small and large intestines. Most cases are infectious, although gastroenteritis may occur after ingestion of drugs and chemical... read more is usually caused by viruses and transmitted from person-to-person by the oral-fecal route.
Age group primarily affected depends on the virus:
Norovirus: Children and adults
Astrovirus: Usually infants and young children
Adenovirus 40 and 41: Infants
COVID-19: All ages
Local epidemics may occur in children, particularly during colder months.
The main symptoms are vomiting and diarrhea.
No specific treatment is recommended, but supportive care, particularly rehydration, is important.
A rotavirus vaccine that is effective against most pathogenic strains is part of the recommended infant vaccination schedule Recommended Immunization Schedule for Ages 0–6 Years . Hand washing and good sanitation measures can help prevent spread.
Some viruses cause only skin lesions (as in molluscum contagiosum Molluscum Contagiosum Molluscum contagiosum is characterized by clusters of pink, dome-shaped, smooth, waxy, or pearly and umbilicated papules 2 to 5 mm in diameter caused by molluscum contagiosum virus, a poxvirus... read more ); others also cause systemic manifestations or lesions elsewhere in the body.
Transmission is typically from person to person; alphaviruses have a mosquito vector.
At least 5 specific viruses (hepatitis A, B, C, D, and E viruses) can cause hepatitis Causes of Hepatitis Hepatitis is inflammation of the liver characterized by diffuse or patchy necrosis. Hepatitis may be acute or chronic (usually defined as lasting > 6 months). Most cases of acute viral hepatitis... read more ; each causes a specific type of hepatitis. Hepatitis D virus can infect only when hepatitis B is present. Transmission is from person to person by contact with infected blood or body secretions or by the fecal-oral route for hepatitis A and E (genotypes 1 and 2).
Other viruses can affect the liver as part of their disease process. Common examples are cytomegalovirus Cytomegalovirus (CMV) Infection Cytomegalovirus (CMV, human herpesvirus type 5) can cause infections that have a wide range of severity. A syndrome of infectious mononucleosis that lacks severe pharyngitis is common. Severe... read more , Epstein-Barr virus Infectious Mononucleosis Infectious mononucleosis is caused by Epstein-Barr virus (EBV, human herpesvirus type 4) and is characterized by fatigue, fever, pharyngitis, and lymphadenopathy. Fatigue may persist weeks or... read more , and yellow fever Yellow Fever Yellow fever is a mosquito-borne flavivirus infection endemic in tropical South America and sub-Saharan Africa. Symptoms may include sudden onset of fever, relative bradycardia, headache, and... read more virus. Less common examples are echovirus, coxsackievirus, and herpes simplex Herpes Simplex Virus (HSV) Infections Herpes simplex viruses (human herpesviruses types 1 and 2) commonly cause recurrent infection affecting the skin, mouth, lips, eyes, and genitals. Common severe infections include encephalitis... read more , rubeola, rubella Measles, Mumps, and Rubella (MMR) Vaccine The measles, mumps, and rubella (MMR) vaccine effectively protects against all 3 infections. People who are given the MMR vaccine according to the US vaccination schedule are considered protected... read more , and varicella Chickenpox Chickenpox is an acute, systemic, usually childhood infection caused by the varicella-zoster virus (human herpesvirus type 3). It usually begins with mild constitutional symptoms that are followed... read more viruses.
Most cases of encephalitis Encephalitis Encephalitis is inflammation of the parenchyma of the brain, resulting from direct viral invasion or occurring as a postinfectious immunologic complication caused by a hypersensitivity reaction... read more are caused by viruses. Many of these viruses are transmitted to humans by blood-sucking arthropods, mainly mosquitoes and ticks; these viruses are called arboviruses Overview of Arbovirus, Arenavirus, and Filovirus Infections Arbovirus (arthropod-borne virus) applies to any virus that is transmitted to humans and/or other vertebrates by certain species of blood-feeding arthropods, chiefly insects (flies and mosquitoes)... read more (arthropod-borne viruses). For such infections, prevention includes avoiding mosquito and tick bites.
Certain viruses cause fever and a bleeding tendency. (See also Arbovirus, Arenavirus, and Filovirus Infections .)
Transmission may involve mosquitoes, ticks, or contact with infected animals (eg, rodents, monkeys, bats) and people. Prevention involves avoiding the means of transmission.
Cutaneous or mucosal infections
Transmission of herpes simplex virus infection Herpes Simplex Virus (HSV) Infections Herpes simplex viruses (human herpesviruses types 1 and 2) commonly cause recurrent infection affecting the skin, mouth, lips, eyes, and genitals. Common severe infections include encephalitis... read more and human papillomavirus Warts Warts are common, benign, epidermal lesions caused by human papillomavirus infection. They can appear anywhere on the body in a variety of morphologies. Diagnosis is by examination. Warts are... read more is by person-to-person contact.
Some viruses cause skin or mucosal lesions that recur and may become chronic.
Enteroviruses Overview of Enterovirus Infections Enteroviruses, along with rhinoviruses (see Common Cold) and human parechoviruses, are a genus of picornaviruses (pico, or small, RNA viruses). All enteroviruses are antigenically heterogeneous... read more , which include coxsackieviruses and echoviruses, can cause various multisystem syndromes, as can cytomegaloviruses Cytomegalovirus (CMV) Infection Cytomegalovirus (CMV, human herpesvirus type 5) can cause infections that have a wide range of severity. A syndrome of infectious mononucleosis that lacks severe pharyngitis is common. Severe... read more .
Transmission of enteroviruses is usually by direct contact with respiratory secretions or stool.
Nonspecific febrile illness
Some viruses cause nonspecific symptoms, including fever, malaise, headaches, and myalgia. Transmission is usually by an arthropod vector. (See also Arbovirus, Arenavirus, and Filovirus Infections .)
Rift Valley fever is usually transmitted through contact with blood, body fluids, or tissues of infected animals, mainly livestock such as cattle, sheep, goats, buffalo, and camels. Transmission can also be through bites from infected mosquitos (see Centers for Disease Control and Prevention: Rift Valley Fever Transmission). Rift Valley fever rarely progresses to ocular disorders, meningoencephalitis, or a hemorrhagic form (which has a 50% mortality rate).
1. Mahase E: Coronavirus covid-19 has killed more people than SARS and MERS combined, despite lower case fatality rate. BMJ 368:m641, 2020. doi: 10.1136/bmj.m641
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