Naturally Occurring Infection

Humans are the only natural reservoirs of variola virus. Person-to-person transmission of smallpox occurs by aerosol droplets expelled from the oropharynx of infected persons, or by direct contact with an infected person. The virus can also be spread through contaminated bedding and clothing.

Despite the fact that smallpox is less contagious than influenza or measles, it is still considered a highly contagious disease. The infectious dose 50, the amount of agent inhaled in aerosol form that is required to cause manifest illness in 50% of susceptible humans, is less than 100 viral particles. The patient can be already infectious in the last day of incubation period, and remains contagious until the scabs separate. The virus can remain viable for months on objects from the victim's surroundings. Though smallpox is spread most readily during dry, cool winter months, the disease can be transmitted in any climate and in all parts of the world.

Physical protection, early vaccination, and isolation of infected patients are the only effective protection against the disease. Patients vaccinated within 2 or 3 days of exposure, will most likely not develop the disease. Vaccination 4 or 5 days after exposure may significantly reduce mortality, but will less effectively reduce morbidity.

Contents: Smallpox (Extensive Information)              Next Topic: Classification and Etiology