Jennifer Millar keeps trash bags and hand sanitizer near her tent, and she regularly pours water mixed with hydrogen peroxide on the sidewalk nearby. Keeping herself and the patch of concrete she calls home clean is a top priority. But this homeless encampment off a Hollywood freeway ramp is often littered with needles and trash and soaked in urine. Rats occasionally scamper through, and Millar fears the consequences. Infectious diseases—some that ravaged populations in the Middle Ages—are resurging in California and around the country, and are hitting homeless populations especially hard. Los Angeles recently experienced an outbreak of typhus—a disease...