"How on earth do the forests survive with all of the wild animals ignoring our advice?"

Most parasites are fairly host-specific. Giardia is a major exception, and affects many species.

Many diseases are species-specific. Salmonella typhi (typhoid fever) is found only in humans. Liquified stools running into a water source could cause an outbreak among the humans in the area. It wouldn't mean a thing to animals.

People in the 'olden days' were probably infected/infested with a lot of things, they were just able to tolerate them better than we can.

Just play it safe and be careful.

Sue