TOP: Wendy Kaler, a microbiologist, has been hired by hospitals and cities across the country to help assess and reduce the risk of an infectious disease outbreak. Kaler, who has been doing this work for 26 years, is concerned the prevalence of feces along streets and sidewalks in San Francisco is leaving the city vulnerable to the spread of diseases. BOTTOM LEFT:MIDDLE: A simple but effective way to avoid disease, says Kaler, is to take off your shoes when you walk inside your home. BOTTOM RIGHT:BOTTOM: While not visible to the naked eye, potentially dangerous bacteria can cling to the bottom of shoes, as depicted in this illustration.