Certainly there will be enough to survive to reproduce. It is easy to see that insects would lose this numbers game against bacteria. It takes a fly two weeks to develop from an egg into a fertile adult. It takes a bacterium 20 minutes to divide. Without immune defenses, an insect would be overcome by bacteria in a matter of hours.
Insects have a variety of immune defenses they use to fight infections.
These are all considered innate immune responses as the insect cannot learn what is pathogenic. This information appears to be programmed into the insect genome. Insects have no immune memory. They do not produce antibodies and thus cannot be vaccinated against disease. This by no way means that insects cannot produce a strong fight against infectious agents.