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.)
Respiratory infections
Upper respiratory infections are likely the most common viral infections. Respiratory infections 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 (A and B), H5N1 and H7N9 avian influenza A viruses, parainfluenza viruses 1 through 4, adenoviruses, respiratory syncytial virus A and B and human metapneumovirus, rhinoviruses, and coronaviruses.
Several coronaviruses cause a respiratory infection that can be severe. In 2002 and 2003, an outbreak of severe acute respiratory syndrome (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 (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) emerged in Wuhan, China and spread in a worldwide pandemic that has killed millions of people (1).
Gastrointestinal infections
Gastroenteritis is usually caused by viruses and transmitted from person-to-person by the oral-fecal route.
Age group primarily affected depends on the virus:
Rotavirus: Children
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.
Centers for Disease Control and Prevention Child and Adolescent Immunization Schedule by Age. Hand washing and good sanitation measures can help prevent spread.
Exanthematous infections
Some viruses cause only skin lesions (as in molluscum contagiosum); others also cause systemic manifestations or lesions elsewhere in the body.
Transmission is typically from person to person; alphaviruses have a mosquito vector.
Hepatic infections
At least 5 specific viruses (hepatitis A, B, C, D, and E viruses) can cause hepatitis; 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, Epstein-Barr virus, and yellow fever virus. Less common examples are echovirus, coxsackievirus, and herpes simplex, rubeola, rubella, and varicella viruses.
Neurologic infections
Most cases of encephalitis 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 (arthropod-borne viruses). For such infections, prevention includes avoiding mosquito and tick bites.
Hemorrhagic fevers
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 and human papillomavirus is by person-to-person contact.
Some viruses cause skin or mucosal lesions that recur and may become chronic.
Multisystem diseases
Enteroviruses, which include coxsackieviruses and echoviruses, can cause various multisystem syndromes, as can cytomegaloviruses.
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).
Reference
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
