Don't confuse chickens carrying salmonella in their intestines with actually falling ill from it. In fact, you likely have a scant amount of salmonella in your intestines right now along with other bacteria you don't really want to know about, and your immune system is keeping them from multiplying and overwhelming your system. Most people who get sick are either babies or old people or someone that has eaten food contaminated with so much salmonella, they end up sick from it because the number of bacteria overwhelms their system. They throw up and have diarrhea and cramps and run a fever.
Chickens that have a low functioning immune system might not be able to fight off the bacteria and they will start to act sick. Watch for a lethargic chicken, hanging out by the fence or facing a wall away from the others, tail held low and flat and feathers fluffed up. They may have very runny poop and won't feel like eating. If you see that, start that chicken on amoxicillin or penicillin.
