Types of Viral Disorders
Categorizing viral infections by the organ system most commonly affected (eg, lungs, GI tract, skin, liver, CNS, 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.
The most common viral infections are probably URIs. Respiratory infections are more likely to cause severe symptoms in infants, the elderly, and patients with a lung or heart disorder.
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, human metapneumovirus, and rhinoviruses (see Some Respiratory Viruses and Professional.heading on page Respiratory Viruses). In 2012, a novel coronavirus, Middle East respiratory syndrome coronavirus (MERS-CoV—see Middle East Respiratory Syndrome (MERS)), appeared in Kuwait; it can cause severe acute respiratory illness and is sometimes fatal. Respiratory viruses are typically spread from person to person by contact with infected respiratory droplets.
Some Respiratory Viruses
Gastroenteritis is usually caused by viruses (see Gastroenteritis) and transmitted from person-to person by the oral-fecal route. Age group primarily affected depends on the virus:
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 (see Table: Recommended Immunization Schedule for Ages 0–6 yr). Hand washing and good sanitation measures can help prevent spread.
Some viruses cause only skin lesions (as in molluscum contagiosum and warts—see Viral Skin Diseases); others also cause systemic manifestations or lesions elsewhere in the body (see Some Exanthematous Viruses). Transmission is typically from person to person; alphaviruses have a mosquito vector.
Some Exanthematous Viruses
At least 5 specific viruses (hepatitis A, B, C, D, and E viruses) can cause hepatitis; each causes a specific type of hepatitis (see Viral Hepatitis and see 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.
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.
Most cases of encephalitis are caused by viruses (see Some Neurologic Viruses and Brain Infections). Many of these viruses are transmitted to humans by blood-eating arthropods, mainly mosquitoes and ticks (see Arboviruses, Arenaviridae, and Filoviridae); these viruses are called arboviruses (arthropod-borne viruses). For such infections, prevention includes avoiding mosquito and tick bites.
Some Neurologic Viruses
Certain viruses cause fever and a bleeding tendency (see Some Viruses That Cause Hemorrhagic Fever and Arboviruses, Arenaviridae, and Filoviridae). Transmission may involve mosquitoes, ticks, or contact with infected animals (eg, rodents, monkeys, bats) and people. Prevention involves avoiding the means of transmission.
Some Viruses That Cause Hemorrhagic Fever
Some viruses cause skin or mucosal lesions that recur and may become chronic (see Some Viruses That Cause Recurrent or Chronic Skin or Mucosal Lesions). Mucocutaneous infections are the most common type of herpes simplex virus infection (see Herpes Simplex Virus (HSV) Infections). Human papillomavirus causes warts (see Warts); some subtypes cause anogenital and oropharyngeal cancer (see Genital Warts and Cervical Cancer). Transmission is by person-to-person contact.
Some Viruses That Cause Recurrent or Chronic Skin or Mucosal Lesions
Enteroviruses, which include coxsackieviruses and echoviruses (see Enteroviruses), can cause various multisystem syndromes, as can cytomegaloviruses (see Some Viruses That Cause Multisystem Disease and Cytomegalovirus (CMV) Infection). Transmission is by the fecal-oral route.
Some Viruses That Cause Multisystem Disease
Some viruses cause nonspecific symptoms, including fever, malaise, headaches, and myalgia (see Some Viruses That Cause Nonspecific Acute Febrile Illness and see Table: Arbovirus, Arenavirus, and Filovirus Diseases). Transmission is usually by an insect or arthropod vector.
Rift Valley fever rarely progresses to ocular disorders, meningoencephalitis, or a hemorrhagic form (which has a 50% mortality rate).