everyone should close the lid BEFORE the flush:
"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."
http://www.straightdope.com/classics/a990416.html