Salmonella is a genus of enterobacteria (Kingdom: Bacteria, Phylum: Proteobacteria, Class: Gamma Proteobacteria, Order: Enterobacteriales, Family: Enterobacteriaceae, if you're a stickler for detail), named after Daniel Elmer Salmon, an American veterinary pathologist, who is credited as its discoverer (some sources say it was actually his research assistant, Theobald Smith, that did the discovering.) The Salmonella genus contains a motley crew of two species, several sub species and more than 2,000 serovars (groupings of microorganisms based on their cell surface antigens).
The bacteria cause salmonellosis, an infection that bestows upon you the joys of diarrhea, abdominal cramps, nausea, vomiting, fever and headaches. These symptoms usually appear within 8-72 hours after infection and can last 4-7 days. The source of infection is usually tainted food; after ingestion, Salmonella binds to the wall of the intestine, penetrates it and then starts to wreak havoc (a more in-depth explanation, if you don't mind some technical terms, can be found here).
So how did this stuff get into tomatoes?
"¢ Contaminated water was used to irrigate the tomato crops "¢ Contaminated animal manure was used as fertilizer "¢ Somewhere, someone involved in the process of picking, packing, processing and preparing the tomatoes didn't wash their hands after using the bathroom "¢ Farm or wild animals spread the bacteria onto tomato crops either with their own feces or feces they tracked in from somewhere else (the 2006 E. coli problem with spinach was traced to a pack of wild boars that stepped in contaminated cow manure and brought it into a spinach field when they grazed there)
Once Salmonella was on the tomatoes (which isn't too much of a problem, since they get rinsed with chlorinated water after harvest), it could have gotten inside through cuts or scars on the skin or through the scar left when tomato is picked from the vine.
As for the source, that's a bigger problem. Pick a vegetable, any vegetable, from your fridge. Do you know where it was grown? Probably not unless you're buying it at a farmers' market directly from the folks who grew it. There's a fairly complex supply chain between the farm and your fridge (example diagrams here and here); the processors along that chain are only required to know who they bought from and who they sold to, and most don't know who is further up or down the chain.
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