A food poisoning outbreak tied to McDonald’s Quarter Pounders was caused by a common type of E. coli bacteria that can cause serious illness and death. Particularly vulnerable are young children, older people or those with weakened immune systems. About 50 people have fallen ill in the outbreak, and one has died. Their ages range from 13 to 88. The bacteria are common in the environment, including water, food and in the intestines of animals and humans. Symptoms typically occur within two days of eating tainted food and include fever, vomiting and diarrhea.