Normal gut strains suppress the growth of harmful bacteria and synthesise vitamins. But some strains are deadly. This last condition is an illness affecting children less than 5 years old. The bacterium Listeria monocytogenes occurs commonly in the environment, both in soil and water. Freezing does not appear to have any detrimental effect.
Listeriosis is uncommon in healthy people.
It is a serious condition, sometimes fatal, in older people and people with deficient immune systems. It can result in miscarriage in pregnant women. Scientists estimate that less than a thousand Listeria bacteria can cause illness in susceptible people. The bacterium Salmonella is a common cause of food-poisoning. There are more than 2000 different strains of Salmonella .