A study has revealed that potentially fatal varieties of common food bug listeria are "uniquely adapted" to infect the heart.
Researchers believe the listeria strains are most likely to affect people with pre-existing heart problems, as well as those who have had heart valve replacements in the past.
US scientists discovered that mice infected with the strains had as much as 15 times more bacteria in their hearts than those exposed to other forms of listeria.
It is hoped that individuals most at risk could be protected by developing new ways of identifying the strains.
Listeria monocytogenes is widespread in the environment, found mainly in vegetation and soil.
It also exists harmlessly in the guts of at least 5% of healthy people.
The bug can grow in low temperatures and can be found in soft cheeses, cold meat products, raw vegetables, fish, salads and unpasteurised milk.
Listeria infections may cause nothing worse than flu-like symptoms and an upset stomach or lead to serious illness involving the blood and nervous system.
Pregnant women are especially susceptible to listeria, which can cause them to miscarry.
Nancy Freitag, from the University of Illinois, Chicago, explained how a subset of infections followed a different path.
"A significant number - about 10% - of L. monocytogenes infections involve the heart," she said. "In these cases, death rate from cardiac illness is estimated to be up to 35%, yet very little is known about how these bacteria infect heart tissues."