How does Salmonella cause disease?
After Salmonella is eaten it passes through the stomach to the intestine. Here, it binds to the wall of the intestine, and through some special proteins that it makes in response to the particular conditions in the intestine it actually penetrates the barrier between us and the outside. Once it has gained access to our insides, it is taken to the liver or spleen. For most other bacteria, this journey would kill them, however Salmonella has evolved mechanisms to prevent our immune system from doing its job efficiently. In the liver, the Salmonella can grow again, and be released back into the intestine.
Of course, not all of the Salmonella pass through the intestinal wall, and many of them are expelled from the intestine in the diarrhea. In regions with poor sanitation, these bacteria can than survive in the soil or in rivers and infect the next person, cow, chicken or mouse that comes along.