The first evidence that the evolution of A. marginale was linked to ecological traits affecting tick vector performance. Some A. marginale strains have evolved under conditions that support pathogen biological transmission by R. microplus, under different ecological traits which affect performance of R. microplus populations. The evolution of other A. marginale strains may be linked to transmission by other tick species or to mechanical transmission in regions where R. microplus is currently eradicated. The information derived from this study is fundamental toward understanding the evolution of other vector-borne pathogens. Ticks are famous for the damage they cause and the diseases they transmit. But all is not negative, and apart from the fact that they consider (if an anthropomorphism is allowed) their hosts as useful for themselves, some aspects of ticks are also useful in medicine. Proteins in the saliva of the American tick Amblyomma cajennense seem to be able to kill tumorous cells in vitro as well as in rats, while leaving normal cells unharmed.