A return to the climate of the middle Pliocene is probable within this century and the increased temperatures, particularly at higher latitudes, will boost the spread of several bacterial, viral and protozoan parasite pathogens which cause severe and/or lethal diseases. The warmer climate effects on the spread and evolution of some fungal pathogens may also increase their virulence, although probable, this is less certain. The increase in the range and pathogen populations in the various hosts and vectors during a return to the middle Pliocene epoch will also increase the probability that several pathogens will evolve more immuno-evasive genetic strains which will be more dangerous for humans and animals. There could also be severe impacts on livestock and agricultural food chains.