Geographical distribution of the viral infections depends on (a) the presence of vectors for the transmission of disease, (b) cofactors facilitating the transmission of disease, and (c) the presence of susceptible population in the particular area. For example, the presence of suitable arthropod vectors for a particular virus in large numbers increases the risk of high prevalence of viral disease in that community.
Viral diseases also show seasonal differences; for example, enteric viruses are more prevalent in the summer due to poor hygiene and use of contaminated water because of shortage of water supply. Similarly, respiratory viruses are more prevalent during winter due to overcrowding, which facilitates the spread of the viruses. The viral infection in a community may occur as an (a) outbreak, (b) epidemic, or (c) as pandemic:
· The outbreak of a viral disease usually occurs from a com-mon source, for example, infected food, and is seen only in clusters of people.
· Epidemics of viral infection, in contrast, occur in a largegeographical area. An epidemic usually occurs due to intro-duction of a new strain of virus into an immunologically susceptible population, e.g., influenza epidemic.
· Pandemics are usually worldwide epidemics resulting fromthe introduction of a new virus, e.g., HIV. Pandemics of influenza have occurred as a result of introduction of new strains of influenza viruses.
Infected humans are usual reservoirs of viruses. They serve as source of infection for other susceptible hosts (HIV, poliomy-elitis, etc.). For many other viral diseases, animals also act as reservoirs. Viral infections transmitted from these animal reser-voirs to humans are called zoonotic viral diseases; for example, rabies is a zoonotic viral disease, and the infection is transmit-ted from the infected dogs, bats, foxes, cats, etc., to humans.
Mosquitoes, ticks, and sand flies are the arthropods that act as vectors for transmission of togaviruses, flaviviruses, bunyaviruses, and reoviruses. These viruses, hence, are called as arboviruses (from arthropod-borne viruses).
Asymptomatic patients are the important source of infection. In HIV and varicella zoster virus infections, the viruses are usually released before the manifestations of the symptoms. Viruses changing the antigenic structure of their genome, e.g., influenza and HIV, infect a large number of immunologically naive population. Rhinovirus and other viruses with many different types similarly cause infection in a large number of susceptible persons.
The mode of transmission of virus depends on (i) the site of viral replication and secretions and (ii) presence or absence of envelope in the viruses:
1. The site of viral replication and secretions: The virusesthat replicate in the intestinal tract are excreted in the feces and are transmitted by fecal–oral route (e.g., picornavirus and reovirus). The viruses that replicate in the respiratory tract, such as influenza virus, are secreted in aerosol drop-lets, hence are transmitted by inhalation.
2. Presence or absence of envelope in the viruses: Theenveloped viruses are relatively fragile viruses, whichrequire presence of intact envelope for their infectivity. These viruses are usually transmitted by respiratory drop-lets, saliva, mucus, blood or semen, organ transplantation, and by ingestion. Many of the enveloped viruses, since they are sensitive to the presence of acid and detergent, are destroyed rapidly in the gastrointestinal tract of the infected humans and hence are not transmitted by fecal– oral route. Hepatitis B virus and coronavirus are excep-tions, which are transmitted by fecal–oral route.
Nonenveloped viruses are relatively sturdy viruses. These canresist drying, extremes of pH and temperature, and the effects of detergents. These nonenveloped viruses, therefore, with-stand the acidity of the stomach and lytic effect of bile in the intestine as well as they withstand mild disinfectants and insufficient sewage technique. Therefore, these viruses are gen-erally transmitted by the respiratory and fecal–oral routes and also often by contaminated objects (fomites). Hepatitis A virus is a common example of enveloped virus transmitted by fecal– oral route. Rhinoviruses and many other nonenveloped viruses are the examples of viruses that are transmitted by contact with fomites, such as handkerchief, towels, bed linens, etc.
The viral diseases can be prevented and spread of viruses can be controlled by (a) adoption of good hygiene, (b) by control of the vectors, and (c) by immunization of the population. Immunization of the population by vaccines is the best means for the control of many diseases. Vaccines are available against many viral diseases (poliomyelitis, rabies). These protect the population against infection by viruses.