Career
His early studies were on the parasitic worms, such as the Old World hookworm, Ancylostoma; the intestinal roundworm, Ascaris; the tapeworm, Taenia, and the threadworm, Filaria. His other investigations were on the life habits and the social life of the termite; the effects of the Phylloxera insect on the grape vines; and a study of the marine arrow worms, the phylum Chaetognatha. In 1886 he described the Koenemia mirabilis, a new order of arachnids.
In 1898 he demonstrated that mosquitoes in the genus Anopheles carried the parasite in the digestive tract and were the vectors transmitting malaria to man. Sir Ronald Ross had also identified these mosquitoes as the transmitting agents; he demonstrated the life cycle of the parasite in the digestive tract of the mosquito. Later, Sir Patrick Manson proved that the bite of a malaria-infected mosquito transmitted malaria to man. The combined findings of Grassi, Laveran, Ross, and Manson led to the campaign which greatly reduced the incidence of malaria in Europe, particularly in Italy.