Faucets that electronically sense the presence of a hand without a touch are sometimes used to cut down on the risk of bacterial contamination. But researchers who ran bacterial tests discovered that half of water samples taken from the electronic faucets grew Legionella, the type of bacteria that cause Legionnaire's disease.
Perhaps even more surprisingly, only 15 percent of water samples collected from manual faucets contained the same germs.
According to NPR:
"Previous research had suggest[ed] that water pooling inside the mechanisms of the automatic faucets could provide a nice home for nasty bugs. The [researchers] found that was true when they took some apart. The electronic faucets have more parts and places bacteria can grow, even if the faucets are flushed regularly."