There are four phases that occur when a bacteria enters an area:
1) Adaptation: this is when they adapt to the environment
2) Log phase: This is when they grow exponentially and cause infection
3) Stationary phase: This is when they have finished growing as much as they can and their population remains steady.
4) Death phase: This is when they run out of nutrients and die.
Keep in mind though that there are more infectious agents than just bacteria. There are also viruses, protozoans, helminthes, prions etc. They all behave somewhat differently.