This one-hundred-and-fourth volume of the IARC Monographs on the Evaluation of Carcinogenic Risks to Humans is the eighth volume devoted to infectious biological agents. Several have been recognized as major risk factors involved in the burden of cancer. In Volume 104, the IARC Monographs consider the following agents for the first time: malaria (a disease caused by infection with the Plasmodium parasite) and four polyomaviruses: the simian virus SV40, and the BK, JC, and Merkel cell polyomaviruses. Infection by these microorganisms concerns a very large proportion of the world population.