How Many Animal Species Have Become Extinct Due to Disease?

I’m not asking about species which were already endangered, and were finally killed off by disease, but of species which were doing fine and dandy, until a new disease appeared and killed them all off. I’m sure that there hasn’t been that many, but I assume that there must be some.

There may not b any using the criteria you specify.

Under normal circumstances if a species is at sustainable breeding levels, ie not endangered already, then it is almost impossible for disease to exterminate it. No matter how virulent and fatal a disease there will always be some individuals in a genetically diverse population that will be resistant. One need only look at the experience of various New World human populations faced with horrible new diseases such as small[ox and influenza to see how true this is. The diseases decimated these populations, but some people survived, and left to their own devices would rapidly have repopulated the area with resistant offspring. The same situation has been observed in the use of potent biological control agents, most famously myxomatosis for rabbit control in both Australia/New Zealand and in Europe…
Disease is only likely to exterminate a population if there is insufficient genetic diversity to permit the survival of sustainable numbers of resistant individuals. Although this can occur through extreme inbreeding (as is feared for cheetahs) the most likely cause is simply a lack of numbers. Essentially the species must be ‘
pre-endangered’ for disease to be the final killer.

The best example I can think of for a disease eliminating an otherwise viable population is the loss of the Thylacine from Australia. Although the species was already under intense pressure from direct human culling and from feral dogs, it existed in huge areas where humans never ventured and where dogs are still unheard of even today. Despite little being known of the species, it should have been able to hang on quite comfortably in these areas. Yet it went into rapid decline early last century and vanished entirely. Disease seems the most likely culprit, albeit assisted heavily by human intervention.

According to this Q & A (wherein a question very similar to the OP’s is asked), some potential beasties driven to extinction by disease would be the Central American golden toad, and most (all?) the native rodents of Christmas Island. I would think that the most likely candidates for extinction-by-disease would be small, island populations. In such populations, even though they may be initially “healthy”, they will typically also be small (depending on the size of the island itself) and specialized.