Health officials are investigating an outbreak in Wellesley of E. coli, a bacterial infection that causes stomach distress and can be serious. Wellesley public health director Leonard Izzo said officials believe that three people have been infected, two seriously. At least one of those infected has been hospitalized, he said. State health officials are trying to pinpoint the source of the infection. No other cases have been reported, they said.Kay Lazar can be reached at firstname.lastname@example.org.