As I understand the technique, you put a food item in a plastic bag and dump it into warm water for some amount of time.
What happens to bacteria growth inside the bag?
If you do it properly, no there are no health issues.
A combination of two things are required to kill beasties: heat and time. The higher the heat you expose them to, the faster they die. Generally any temperature above 130 F is enough to kill most beasties, it just requires a significantly larger amount of time compared to a higher temperature.
I go into a lot of detail regarding Salmonella in this answer of mine:
I have several tables there to show exact timings, etc.
If you think I should copy/paste that here, then let me know in a comment
There are a couple of things to consider when cooking sous vide in terms of safety:
Douglas Baldwin has produced an excellent, free resource, A Practical Guide to Sous Vide, that goes in to great length on all the safety concerns. It also lists general minimum time per temp per width for different cuts of meat and discusses when you can ignore those and what the additional risks and considerations are.