Acute infection

This is the pattern observed during many viral infections in otherwise healthy individuals. Viral replication leads to cellular damage and clinical symptoms of varying severity followed by virus eradication by the immune system and recovery, accompanied by a level  of immunity to  the  virus.  This is true of most viral infections encountered during childhood such as measles, mumps, chickenpox, and German measles, and most respiratory viral diseases for which a number of viruses are responsible. Indeed a broad spectrum of other viruses also follows this pattern, including hepatitis A virus (infectious hepatitis), rotavirus (gut infections), and Coxsackie virus (myocarditis, pericar- ditis, conjunctivitis). Poliovirus, which we have seen leads to an inapparent infection within the gut, may also replicate within the central nervous system. Here, virus replication and cell damage cause clinical symptoms; an acute infection. Until  the  develop- ment of excellent vaccines within the  1950s,  polio  was  a severe, worldwide, and  often fatal  disease due  to the  muscular paralysis that resulted from  viral  infection of motor neurons.

Many viruses are fatal in distinct circumstances or in a percentage of victims, but some are always fatal, including cerebral rabies and HIV infections. Following the bite of a rabid animal, the rabies virus replicates within the peripheral nerves and during an incubation period of between 30 and 90 days it moves to the spinal cord and brain. During this time it is possible to treat the patient to prevent the virus reaching the central nervous system, but if it does so then replication here results in release of more viruses that then spread to virtually all the tissues of the body including the salivary glands where it is transmitted in the saliva. In only a short time (7–12 days) from this point, the patient develops a vari- ety of abnormalities including hydrophobia (aversion to water), rigidity, photophobia (aversion to light), focal or generalized convulsions, and a variety of autonomic distur- bances that precede a flaccid paralysis, coma, and death.

Acute and inapparent infections are characterized by their limited duration, as a result of the successful intervention of the host immune response. However, some viruses have ways of avoiding immune clearance, such that they are not totally eliminated from the body. These viruses persist in the host in one form or another, and give rise to long-term infections.