Flies perch on animals food but they never get affected by it. Why i s human case different?
When our food get exposed to flies, they perch on it and lay their eggs or deficate on it which is harmful to human health. Animals eat what is far worst than that but still remain healthy. What could be the reason?

You're literally comparing health effects between animals which live 5-15 years and animals [humans] who live 70-120 years.

When a living organism is exposed to health hazards over time, the longer the exposure, the more severe the effects. 

Also, animals get sick from all the crap they eat, the thing is, animals evolved to not show any symptoms of illness. 

There's a saying, to be healthy as a horse, but horses health is extremely fragile.

They just don't exhibit symptoms. 

They just suddenly get weak and die in a span of a week.

Humans can sustain injuries and disease lethal to almost any other animal, and survive.