As far as bacteria are concerned, other living creatures are just another niche to exploit, which means that pretty much every animal and plant has a set of bacterial pathogens that come along with it. These bacteria have made the animal in question their speciality, and are highly adapted to live inside their hosts. While these bacteria often make the host ill, or less fit, or sometimes dead, the longer they live with their host, overall, the less they damage it. After all, it’s no help to the bacteria if their home drops down dead right after they’ve moved in.
A great example of this appeared in PLoS Pathogens this month (reference 1), concerning the bacteria Wolbachia. These bacteria infect insects and other arthropods and are much beloved of journalists (well, compared to other insect bacteria at least) because one of their effects is to stop insects producing male offspring (so only female survive to pass on the bacterial genome), which gives journalists the opportunity to write silly headlines.