Malaria, caused by the parasite Plasmodium,ranks fifth among all infectious diseases as a cause of mortality worldwide. It is an overwhelming public health problem and causes one or two million deaths every year. Eradication programmes, most notably by a DDT spraying programme in the 1950s and 1960s, were attempted but did not succeed in any long lasting way. The incidence of malaria today is far greater than it was 40 years ago. The reasons include resistance of the mosquito vector to DDT, and of the malarial parasite to the drug chloroquine, the most common anti-malarial in use for the last half century. Severe malaria is fatal if untreated or when improperly treated with chloroquine. Dengue fever, which is caused by a virus, is the other major problem. After starting in East Asia in the 1950s, the epidemic reached India in the 1990s in a major way. Dengue shock syndrome and dengue haemorrhagic fever reflect increased transmission and can be fatal. Unlike malaria, there is no effective therapy against dengue. Filariasis is caused by the worms Wuchereria and Brugia resulting in fever and swelling of the limbs. It requires intense exposure to mosquitoes, unlike malaria and dengue which can be contracted by a single mosquito bite. While not life threatening, it causes disability and chronic illness. Small flies (sandflies) transmit kala azar which has reached epi-demic proportions in Bihar and adjoining Uttar Pradesh. An emerging problem is resistance to anti-mony compounds, the traditional treatment. Japanese encephalitis is caused by a mosquito-transmitted virus. It is common in rice farming areas, especially where pigs abound (these serve as reservoir hosts). The disease can cause coma and death.