There are many mechanism for different bacteria or virus to cause diarrhoea. For Salmonella, they invade the epthelial cells of our gut, impare its function to control water absoprtion, thus leading to the pulling out of water from the body into the gut result in watery stool (diarrhoea). Salmonella also attract white blood cells to kill them when the are in the epithelial cell result in the inflammation of the gut. The diarrhoea usually last for couple of days until the Salmonella themselves being flushed out of the gut and the symptom usually resolved. Water and electrolytes  replacement is important during diarrhoea and medical advice is required to make sure no complication is occured, which could be life threatening.