Smallpox is a severe viral disease caused by the variola virus, which has not existed naturally since the World Health Organisation (WHO) announced the global eradication of smallpox in 1980.
While the disease is not infectious during the asymptomatic incubation period, it is easily spread from person to person once a fever and other symptoms commence. The patient remains infectious until all the pox scabs have disappeared from the skin.
Viruses are expelled from the throat, leading to airborne transmission. Even contaminated clothing can cause new outbreaks of infection. As a result, the WHO estimates that each infected person can cause 10 further infections.
All suspected cases of smallpox must be transferred to a designated isolation unit in a hospital, and patients must remain isolated for a period of three weeks. They must be looked after by professionals who have been vaccinated against smallpox and are using full protective equipment.
The household and contacts of a person diagnosed with smallpox must be vaccinated and quarantined for 18 days. There is no specific treatment for the disease - the most effective counter-measure is vaccination prior to exposure.
Immunising a person in the first few days of the incubation period can lessen the impact of the disease and reduces the risk of death by 50 per cent.
Research suggests that 30 per cent of people who develop smallpox will die. Others will suffer serious complications, such as blindness and swelling of the brain (encephalitis).
The current smallpox vaccine is made from live vaccinia (smallpox) virus. It is given in the upper arm using a special forked needle. A permanent scar means the vaccine has "taken". Newer vaccines are in development and are expected to be available in 2003.
Smallpox vaccine is not suitable for pregnant women, those whose immune systems are compromised or people with eczema.