A number of serious infectious diseases are caused by parasites spread by insects. River blindness is caused by a tiny worm that can infect the cornea and cause blindness. Lymphatic filariasis, or elephantiasis, is also caused by a worm and produces chronic swelling. Satoshi Omura cultured bacteria, which produce substances that inhibit the growth of other microorganisms. In 1978 he succeeded in culturing a strain from which William Campbell purified a substance, avermectin, which in a chemically modified form, ivermectin, proved effective against river blindness and elephantiasis.