This lecture focused on the general problem of infectious diseases, their prevalence, their potential seriousness and the inequality of the world's population in the face of these infections, underlining the fact that mortality from infection remains one of the major causes of death in the world today. All aspects of the virulence mechanisms of microorganisms were reviewed, as well as the host's protective mechanisms. Particular attention was paid to the importance of local immunity and its components at the host/external environment interface (respiratory tract, digestive tract, skin). The lecture then turned to the valuable information provided by genetics, which teaches us that we are not all equal when it comes to infectious risks. Two types of results were discussed: i) the identification of frequent but low-risk genetic variants, and ii) the identification of rare variants creating a high risk of a particular type of infection.
It was then shown how, on a population scale, the study of genetic variations (negative and positive selection of alleles, balanced selection) has made it possible in recent years to establish "signatures" of resistance to pathogens with which man has cohabited for over 100,000 years. Examples have been presented - including recent selection - of genetic variants undoubtedly linked to plague pandemics, including that of the 14th century. This work is now leading to the possibility of joint analysis of variants and their functional consequences in immune response mechanisms (innate and adaptive). The parallel evolution of microbes (host adaptation) has also been illustrated by the study of the evolution over the last 5,000 years of Yersinia pestis, the bacterium responsible for plague. Finally, the effects of evolution on the characteristics of the immune system were shown by discussing the concepts of redundancy of immune responses.
