Comparative medicine has a long and strong tradition, in which studies of naturally occurring diseases in other animals has shed much light on the origins and pathophysiology of human ailments. Less attention has been paid to the flip side of the coin, i.e., are there diseases that are preferentially or uniquely human? In our studies of the comparative biology of humans and other hominids (the so-called "great apes") we have come across several remarkable examples in which common human diseases appear to be either absent or manifest in rather modified form, in these closest living evolutionary cousins. Conversely, examples exist of diseases not found in humans but prominent in another hominids. Given the remarkable genetic similarity of all these species, it is worthwhile investigating these differences, with the goal of better understanding the disease processes involved. This talk will present a summary of available information on this topic, and also consider it in the light of certain genetic and molecular differences in sialic acid biology between humans and other hominids.