Health officials are alerting hospitals and nursing homes in Chicago to an unexplained bacterial outbreak linked to three patient deaths and 35 illnesses. The elderly appear to be most susceptible to the infection, caused by a normally harmless intestinal bacterium, Clostridium difficili, officials said. Unchecked, the bacterium can cause diarrhea, high fever, dehydration and internal bleeding. Officials speculated that the outbreak may be linked to raw sewage, which was dumped into Lake Michigan during a flood caused by unusually heavy rains in mid-August. Chicago gets its drinking water from the lake.