Virus Infection

Viruses are the smallest infectious agents whose genome is an element of either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) but never both, enclosed within an outer shell of protein, capsid. Viruses can reproduce only inside living cells and utilize .the synthetic machinery of the host cell for synthesis of specialized particles, the 'virions', which contain the viral genome and serve as a vehicle to carry the genome to other cells.

When a virion enters its host cell the capsid is stripped off and its nucleic acid is liberated within the host cells entering into an eclipse phase in their reproductive cycle, a characteristic of all true viruses. The surface proteins of the virion have special affinity for specific receptor sites on the host cell. Proteins also contain the viral antigens that stimulate the host's immune responses during infection. Viruses possess the property of haemagglutination.

Interferon, a protein produced by the cell after infection with a virus interferes with the replication of the other viruses. Inactivated virus and also a few other substances (foreign nucleic acids and other synthetic polynucleotides) also induce synthesis of interferon.

Any of the following events may occur following presence of virus within a cell :

1. Destruction, e.g., primary herpes hominis virus infection.

2. Stimulation and. multiplication first forming a papule which then undergoes destruction associated with inflammatory exudate forming vesicle, which subsequently evolves into a pustule and finally drying up to form scab, e.g. variola, varicella.

3. Stimulation of the cells to proliferate indefinitely forming papillomata, e.g. warts.

Skin is one of the principal target organs for viral attack. Rashes (exanthemata) are characteristic of many acute systemic viral infections where virions in the. blood invade the endothelium of the capillaries and venules of the dennis . Viral rash may also be related to antigen-antibody reaction and the circulation of immune complexes. The rash may be an outward sign of the neutralization of circulating virus by antibody. Haemorrhagic rashes may be due to disseminated intravascular coagulation precipitated by the circulating virus antibodies and immune complexes.

In some instances where the primary process of invasion is in the epidermis the skin lesions are characterized by intracellular inclusions and the formation of multinucleate giant cells.