The nation's top disease hunters are racing to solve a mysterious outbreak that is unfolding right in their own backyard.

The death of an infant in New Orleans last week has been linked to at least 10 other cases of E. coli illness in four southern states. The largest cluster of five sickened people, ranging in age from 18 to 52, is centered in Atlanta, home to the Centers for Disease Control and Prevention.

"At this time, we continue to interview new cases as we are notified of them," Georgia's Department of Public Health spokeswoman Nicole Price said in an email to ABC News. "We have detected no food items or environmental exposures that are statistically associated with illness at this time. This investigation is ongoing."

Maelan Elizabeth Graffagnini was 21 months old when she died last Thursday at a hospital in New Orleans. Two others in the New Orleans area were also recently stricken by the same strain of E. coli, known as 0145.

Alabama public health officials have linked two cases to this outbreak. And in Florida, a 22-year-old woman's illness has been traced to the same dangerous bacterium.

Aside from the E. coli strain, all these cases have in common is that officials still have no idea what caused the illnesses.

"The likely exposure is a food source," Louisiana Department of Health spokesman Tom Gasparoli said. "But this has yet to be confirmed. Often, the contact source is not found."

Epidemiologists at CDC headquarters are poring over data sent in from the states in search of a common factor that could pinpoint a cause.

For any E. coli outbreak at this time of year, suspicions immediately turn to undercooked ground beef. The period from April through September is what scientists call "high-prevalence season" for E. coli.

E. coli are a common bacteria and not every strain is dangerous. But some, like those that carry the 0145 genetic fingerprint that is behind this outbreak, produce a deadly toxin known as shiga. This poison can cause violent reactions, including severe kidney damage and death.

Until this week, the government was not checking meat for the 0145 strain. Just this week, the U.S. Department of Agriculture for the first time began testing meat for six new strains of E. coli, including the strain causing this outbreak.

In an unrelated case, a 6-year-old boy in Millbury, Mass., died last week from kidney failure caused by E. coli. Massachusetts health officials said scientists have determined his illness was not caused by the same strain of E. coli as the clusters in the South. Officials in Tennessee said a recent E. coli case in that state was also unconnected.