Pathogenesis

Pathogenicity is the capacity of an organism to cause disease. During virus infections, diseases symptoms arise from two causes, direct injury caused by virus replication and the side effects of the immune response to infection. The balance between these two is variable. The majority of virus infections are asymptomatic and do not cause disease. Only a tiny number of infections cause serious or life-threatening consequences such as immunodeficiency, or tumors. Diseases caused by previously unknown viruses continue to be regularly identified. Occasionally, known viruses appear to change their behavior, suddenly causing outbreaks of diseases. Such viruses are known as emerging viruses and frequently the cause of a new disease is when a virus switches host species and begins to infect another, and changes in human activities also result in the emergence of new or previously unrecognized diseases.