It's been the saviour of many a food mishap: as long as you pick your dropped item up from the floor within five seconds, then it'll be fine (and any longer just stick it on a barbecue and it'll 'burn off').
But now it seems like there is actual, real, scientific evidence to back up this long-held, but much-derided claim. A team of students at Aston University in Birmingham, led by Professor of Microbiology Anthony Hilton, studied the transfer of E. coli and Staphylococcus aureus from a variety of indoor floor surfaces to a variety of food types, over a range of times, from 3 to 30 seconds.
They found that "time is a significant factor in the transfer of bacteria from a floor surface to a piece of food", with the factor positively correlated. In addition, the type of flooring is important: hardwood is bad for bacteria transfer (well, good for the bacteria, but bad for you) whilst a carpet gives you a bit more chance of escaping, infection-free.
Hilton continued, "Consuming food dropped on the floor still carries an infection risk as it very much depends on which bacteria are present on the floor at the time; however the findings of this study will bring some light relief to those who have been employing the five-second rule for years, despite a general consensus that it is purely a myth."