Salmonella is a gram-negative bacteria from the family Enterobacteriaceae. Most commonly known as a cause of enteritis (inflammation of the intestines).
There are almost 2,000 serotypes of the genus Salmonella. They live in the intestinal tracts of many species of mammal, birds, amphibians and reptiles. Salmonella appears to be relatively uncommon in cats. Salmonella in cats is of particular importance because it is a zoonotic disease, ie: one which can be passed from animals to humans.
Salmonellosis causes a reddening of the intestinal mucosa as well as reddening of the mesenteric (membranous sheet attaching various organs to the body wall, especially the peritoneal fold attaching the intestine to the dorsal body wall)  lymph nodes. At this point, the bacteria and or their toxin are either contained by the body's own defences or invades the bloodstream (bacteremia) and then onto other organs such as the liver.
It can cause abortion in pregnant females.
What are the symptoms of salmonellosis?
Most cats infected with salmonella have what's known as "subclinical carrier state". This means that they have been infected but only very mildly and don't display any clinical manifestations of the disease. Salmonellosis isn't seen very often in cats and it is believed they have a natural immunity to the bacteria. Risk factors which can make a cat more susceptible to salmonellosis include cats in high stress situations and environments, cats who's natural resistance has been compromised due to another infection, general poor health status, hospitalised animals and kittens. Kittens are more likely to be clinically affected than adult cats.  Symptoms of salmonellosis appear after 2 - 4 days of exposure and can include.
Salmonella is faeces and sometimes the saliva. It can grow on food and can survive on objects for long periods of time. Cats often become infected from catching prey.
Cats are most commonly infected via contaminated food or catching infected animals. In fact, Salmonellosis in cats has been called "song bird fever" due to it's association with cats acquiring infection from hunted birds.
How is it diagnosed?
Bacterial cultures from rectal swabs or fresh feces.
How is it treated?
If the infection isn't severe then supportive care will be given. IV fluids, electrolytes etc.
Food and water are restricted while vomiting and diarrhoea are present.
It seems there is some controversy over the use of antibiotics to treat simple cases of salmonella enteritis (intestinal inflammation) with diarrhoea in cats, suggesting that antibiotics can actually favour the growth of antibiotic resistant strains of salmonella. Antibiotics (sulfa) are therefore reserved for severely ill cats.
Can I catch salmonella from my cat?
Cats have been found to shed the serotype of salmonella which are pathogenic to humans. However, there are very few (if any) reports of human infection from cats. There is a much, much greater risk of becoming infected via infected food.
When treating a cat with salmonellosis strict hygiene is vital. After handling your cat or it's utensils, you should disinfect your hands immediately.
Bleach is the best method to kill salmonella bacteria in the environment.
 Feline Husbandry - Diseases and Management in the Multiple-Cat Environment - Niels C. Pederson,
 Saunders Comprehensive Veterinary Dictionary - D. C. Blood, V. P. Studdert.