Roland98

Actually a number of studies have shown that this is not necessarily or even usually the case. If a pathogen has a large potential for transmission, for example, it will not be disadvantageous for it to kill its host, or even to do so very quickly in some cases. For example, arboviruses can remain at a high level of virulence, because 1) having a higher titre in the host increases the chances that the arthropod will take up virus particles during a blood meal; and 2) having an incapacitated host makes it less likely that they will respond to the vector during said blood meal. This is only one example; I can post some refs if anyone's interested.

Roland98

We don't have them already? Thing is, there really aren't "plenty" of nearby empty "disease niches." And there must be some kind of selection for those who gain any kind of nasty mutation to survive differentially from their parental strains. Generally, the sheer number of bacteria present allows the normal flora to maintain the "status quo." Of course, when you take antibiotics, this can disrupt it, and if the ones which happen to be resistant also carry virulence genes, things can get nasty rather quickly.

Coragyps

I rather doubt it, except perhaps for televangelists, Rush Limbaugh, and senior government officials. Feces is allegedly 50% + bacteria by mass, and the bacterial cells in our bodies (mostly in our colons) apparently outnumber the eukaryotic cells, but since each only weighs a fractional percentage as much, surely they don't contribute that much mass compared to body weight. With the above exceptions, of course.

SEF

The mass percentage does seem a little off but I haven't seen any such calculations anyway. I suppose they might try to include mitochondria. What seems much more likely is that non-bacterial parasites, eg tapeworms or those wasp larvae in caterpillars, could make up a serious percentage of an animal's body.