Historically, it has tended to appear towards the end of an outbreak of bubonic plague, when the bacterial infection has spread to the lungs. Public health experts are baffled by the outbreak in Surat, which has claimed hundreds of lives.
There has been an outbreak of bubonic plague in Maharashtra state, more than 500 miles away. Epidemiologists are trying to establish if the two outbreaks are related.
The plagues are caused by the bacterium Yersinia pestis, carried by fleas which live on rodents and other wildlife. Social disruption - such as follows an earthquake, flood or some other natural disaster - is usually linked to an outbreak. Equally, an epidemic of the infection among local wildlife may be a factor.
In bubonic plague, the focus of infection is the lymphatic system, part of the immune system, where it can, to some extent, be contained.
In pneumonic plague, however, the lungs are attacked and the infection can spread rapidly via droplets from the mouth and nose. The bacteria produce poisons that destroy cells and disrupt the fragile balance of fluid and salts in the body. The cause of death is usually shock.
The symptoms are flu-like, with a high fever, chills, chest pain, fluid on the lungs and bloody sputum, and in some cases swollen lymph glands. The incubation period - the time between infection and appearance of the symptoms - can be as little as a day. Many victims report cold symptoms and are dead within 24 hours.
Effective treatment with the antibiotics tetracycline, chloramphenicol or streptomycin is cheap and readily available in most parts of India, although early treatment is vital. A vaccine is available.
A small number of cases of pneumonic plague are reported to the World Health Organisation every year. Last year there were cases in Zaire, Madagascar, Peru, Brazil, Mongolia and Zimbabwe, and one in the US, in New Mexico.
Bohumil Drasar, professor of bacteriology at the London School of Hygiene and Tropical Medicine, says that such cases tend to occur in 'single figures' only, and that the infection is very rare.Reuse content