In any case, the Food and Drug Administration’s new egg safety rule, which went into effect in July, after the current outbreak began, applies only to large operations with at least 50,000 hens. (In 2012, it will expand to include farms with at least 3,000 hens.) That means that smaller operations are not required to put in place the same safeguards, like rodent control measures.

The case for cage-free eggs is also subject to debate. Some scientific studies, mostly conducted in Europe, have found lower salmonella rates in flocks of cage-free hens, compared with hens living in cages. But that could be explained by the fact that the cage-free henhouses in the studies were often newer and less likely to harbor the rodents that can spread salmonella.

Why have no deaths been reported in the current outbreak?

A big reason is that the deadly outbreaks in the past often occurred in nursing homes or hospitals, where the bacteria attacked the elderly or infirm. After several deaths in the late 1980s, nursing home and hospital kitchens largely switched to pasteurized egg products.

How does salmonella get into the eggs anyway?

The bacteria infect the laying hens but do not make them sick, so they show no symptoms. Bacteria can then enter a bird’s ovary or oviduct, where they can infect the egg as it is formed. But that’s rare, and even in those cases, only a small number of eggs will be laid with the bacteria inside.

How did this problem start?

Salmonella enteritidis in chicken eggs mysteriously began to appear in many countries at about the same time in the late 1970s and early 1980s. One theory, by Andreas J. Bäumler, a microbiologist at the University of California, Davis, ties the bacterium’s emergence to the virtual eradication of two related strains of salmonella that make chickens sick. Once those strains were stamped out, through culling of infected birds, the theory goes, immunity to similar strains of salmonella decreased. That opened up a niche for enteritidis to thrive.