Health officials say a recent outbreak of salmonella infections in Pennsylvania, Maryland, Ohio, Virginia and West Virginia are probably linked to tainted tomatoes on sandwiches served by Sheetz convenience stores. Here are some basics on salmonella and E. coli, two of the most common germs threatening food:

QWhat are salmonella and E. coli, and how are they spread?

ABoth are bacteria that occur commonly in the intestinal tracts of animals and humans, but some types cause serious illness. They are often spread by the unwashed hands of food workers.

Some strains of salmonella can turn up on fruits and vegetables; others are found in eggs and poultry. E. coli is most often linked to undercooked meat but also can be found in raw sprouts and lettuce. Both bacteria are also found in unpasteurized milk and juice.

What are the symptoms of these illnesses?

People exposed to salmonella can get sick in one to three days, with diarrhea, fever, abdominal cramps and vomiting that last as many as seven days. E. coli's basic symptoms are similar but can take as many as eight days to develop and may last as many as 10 days. Both germs are most dangerous to young children, the elderly and people with weakened immune systems. Deaths are rare, although one strain of E. coli causes particularly severe illness.

What outbreaks have been the worst?

Perhaps the largest salmonella outbreak in the country sickened about 20,000 people in 1994 who ate ice cream from Schwan's, a home food delivery company based in Marshall, Minn. Investigators traced the bacteria to tanker trucks used to haul both ice cream and raw eggs. Over the past decade, hundreds also have become sick from tainted tomatoes and orange juice.

A major E. coli outbreak in 1993 sickened about 700 people and killed four who ate undercooked Jack in the Box hamburgers in Washington state. That outbreak led to tighter U.S. Department of Agriculture safety standards for meat and poultry producers.