I know they can catch some (right?), but what goes on at the cellular level to keep my dog from catching my cold, or my mononucleosis, or some kid’s chicken pox? How about skin disorders like ringworm? Stuff like tuberculosis and influenza?
IANAD, but my understanding is that microbs are fairly particular about the hosts they infect. You won’t catch your cat’s upper respiratory infection and she shouldn’t catch yours. If you were exposed to their version of the virus presumably you cells could combat it easily or more likely their virus won’t have any negative effect on your cells in the first place.
The answers are many and varied.
For viral infections the reason for species specific infections is the difference in cell surface proteins. Viruses need to enter living cells to reproduce and in order to do this they have to chemicaly bond to the proteins on the cell’'s surface. The proteins on the cells of different species are chemically different, and so viruses can’t enter. This accounts for the non-infectivity of chicken poxes, colds, influenza and mononucleosis.
For some bacterial infections like cholera the toxins released also affect specific cell proteins, which means that animals with different protein structures will be immune.
Other bacterial and protozoal diseases, such as syphillus and malaria, are very temperature sensitive. Since most animals have a much higher body temperature than people the microbe is cooked as soon as it enters the body.
Perhaps one of the main reasons infection doesn’t occur is because many infectious microbes and parasites have altered their own surface chemistry to eitehr mimic their host’s body, or neutralise immune responses. Such alterations tend to be species specific and what will fool your immune system won’t fool your dog’s and vice versa.
I don’t know whether dogs can get tuberculosis, but it is a fairly major problem amongst other animals, notably cattle, and the cattle strain can infect humans so it’s certainly not species specific. I don’t know, but since the body temperature of cattle is closer to human I’d WAG that the temperature differential may prevent smaller animals becoming infected. Dogs and cats can both get ringworm, and cats are in fact one of the main reservoirs of the disease. It’s mainly the shorter haired dogs that get infected.
I couldn’t possibly say it better than Gaspode did, though I can’t help mentioning my favorite animal disease factoid…armadilloes are the only other species that get Hansen’s disease (leprosy). It’s caused by a bacterium (Mycobacterium leprae) that’s very temperature-sensitive, & armadilloes’ skin temperature is roughly the same as humans’.
Thanks, Gaspode! The body-temperature deal makes a lot of sense, as dogs and cats regular body temperature is about where a human with a high fever would be (104F ?)-- and fevers occur to fight infections.