Salmonella is an enterobacteriaceae, which loves to grow at around 37oC (our typicalbody temperature), it usually lives quite harmlessly in farm animals, and is pathogenic if it finds itself in a human host. Salmonella food poisoning has been increasing since the 80's, over 4 million people are infected with an intestinal disease in the UK each year (Wheeler, et al., 1999). Salmonella is the second most common food pathogen in Europe, with 1.7 million deaths per year worldwide (usually through causing typhoid), and particularly in the developing world due to poor drinking water. It is interesting to note that whilst cases of typhoid fever has been dramatically reduced from the 1930's to present day, the number of cases of other forms of salmonella (which usually causes gastroenteritis) have been increasing, this is due to poor intensive farming, and lower levels of our cooking standards.
Salmonella usually does not harm us too much, if we do get infected the symptoms include diarrhoea, fever, and abdominal cramps, the symptoms come 3 days after infection. The real danger is to immune-compromised individuals, there are also higher infections in elderly people, and children. Salmonella lives in cattle, chicken, pigs, and pets such as reptiles, and wild birds, infections arise from food contaminated with farces, but they can be killed if cooked at 650C for 10 minutes. Salmonella has been found increasingly in salads, because they are not cooked, and can be in contact with animal feces, so it is not just meat you have to be careful with.
Salmonella has no vaccine, and it is a remarkable organism which has devoted its evolution to evading our immune system, it enters through the gut wall, and actually makes proteins which induce our cells to grab it and take it in to our body. Once inside, they can survive with in vacuoles, and phagocytes (which our body uses to engulf and destroy bacteria), and actually uses our own immune system against us, as we give them a protected place to grow, and spread further through our body. They can survive at low pH within our stomach, and can spread through our lymphatic system in systemic cases. Part of the reason we get diarrhoea from the infection is because our intestine takes up the bacteria, and our immune system detects it, which makes phagocytes, which increases the salmonella survival, which creates an inflammation of our gut mucosa, which eventually kills our cells, which releases fluid causing acute diarrhoea.