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. There are three well characterized arms of the insect immune defense system. Insects have blood cells which can eat invaders, the fat body of the insect can secrete antibiotics into the blood and reactive oxygen can by synthesized to kill invaders. Genetic analysis of the fly has demonstrated a signal transduction cascade exists that uses the receptor Toll to transmit signals to IkB like proteins to turn on an immune response.