Why do flies wont die if they are really dirty and germy insects?

Maybe because bacteria that cause sickness in human doesn't always work on other species, that's why they can live in a place that in human perspective is 'dirty'. Similar with our pets, sometimes, our pet got sick and die but we as human didn't get sick either because the bacteria or germs that attack our pet is not giving any effect on human.

Their immune system can handle them, It may affect them though cos some insects have few days as a lifespan, flies have 21 days as an average lifespan so who knows when they were 19 days old their bodies wont be able to handle those diseases anymore. It may sound like they have a higher immune system than humans but they have shorter lifespan.

Their bodies are built to resist the bacteria and survive in their environment. Put them in a clean room and they might just perish.

yes,even though they are always have germs in their body,i think they live because of that dirty and germs live in their body,they are really annoying because they really love to tap in your food and aside from that they are irritating insect because they always tap in a dirty things.

Did you considered that you also have different germs and bacteria all over your body? Well flies and insects are just immune to most of the micro organisms that they came contact with. Like other animals, do you notice there crocodile that swims in some dirty river if a human swims in their he may get sick after. Did you notice those beggars outside or in the streets, they do not get sick easily because they get immune being expose to many organisms outside. Good day and God bless

They are dirty and germy to us but not to themselves. The environment they live in is ideal for their growth and development but harmful for human beings. It is nothing but a balance in ecology.