From here:
In 1975 Professor Gerba published a scientific article describing the little-known phenomenon of bacterial and viral aerosols due to toilet flushing. The more you learn about it, the scarier it sounds. According to Gerba, close-up photos of the germy ejecta look like “Baghdad at night during a U.S. air attack.” The article ominously depicts a “floor plan of experimental bathroom with location of gauze pads for viral fallout experiments.” A lot of virus fell on those gauze pads, Gerba found, and a lot of bacteria too. In fact, significant quantities of microbes floated around the bathroom for at least two hours after each flush.
As Professor Gerba’s research would later determine, however, the bathroom was hardly the most dangerous part of the house, microbe-wise. The real pesthole: the kitchen sponge or dishcloth, where fecal coliform bacteria from raw meat and such could fester in a damp, nurturing (for a germ) environment. Next came the kitchen sink, the bathroom sink, and the kitchen faucet handle. The toilet seat was the least contaminated of 15 household locales studied. “If an alien came from space and studied the bacterial counts,” the professor says, “he probably would conclude he should wash his hands in your toilet and crap in your sink.”