In school they're lurking on water fountains and keyboards, library books and art supplies. Germs are everywhere, and the start of school means the start of sick days.
Nearly 22 million school days are lost annually due to the common cold, according to the Centers for Disease Control and Prevention.
Your child's No. 1 defense against getting sick in the close confines of the classroom is good handwashing skills.
"Schools are a mixing place," said Joseph Eisenberg, chairman and professor of epidemiology at the University of Michigan School of Public Health. Germs in your home are your own germs or your family's germs. "Your immune system is familiar with them. What comes from the outside can put you more at risk," he said.
At schools germs spread when kids come together and start touching the same doorknobs, wastebaskets and computer screens.
"It's a great opportunity for pathogens to hitch on to a susceptible person," Eisenberg said.
The four types of germs — bacteria and viruses, protozoa and fungi spread easily between people and cause many types of illnesses. High-touch surface areas like desks, the backs of bus seats, sinks, toys, gym and playground equipment, backpacks and lunch bags, cafeteria trays and communal pencil sharpeners can all be breeding grounds for germs.
Depending on the pathogen the germ may be present for a few hours, as is the case for the flu virus, or weeks for norovirus, which is commonly associated with cruise ship outbreaks but can also occur in schools and child care centers.
One place that hasn't been mentioned is the bathroom -- and toilet seats in particular. Because people associate bathrooms and toilets with germs, they tend to be cleaned more frequently, according to Charles Gerba, a professor of microbiology and environmental sciences at the University of Arizona.
"The more cleaning that happens, the less pathogens," Eisenberg said.
Proper handwashing with soap and water is the best way to prevent the spread of illness, Eisenberg said. For good hand hygiene, wash your hands rigorously with soap under running water for 20 to 30 seconds at least.
"Wash hands before and after — wash hands before eating to protect yourself and wash hands after using the bathroom to prevent the spread of illness," Eisenberg said.
Parents should encourage basic hygiene habits when kids are young so it become part of their everyday routine, he said.
If soap and water are not available, the Centers for Disease Control and Prevention say you should use an alcohol-based hand sanitizer that contains 60% to 95% alcohol. Non-alcohol-based hand sanitizers are less effective at killing germs, and even alcohol-based sanitizers don't kill all germs, including norovirus.
While parents want to protect their kids from illness, they shouldn't overdo it and create germaphobes who take it to the extreme and are constantly reaching for a squirt of hand sanitizer. "Nothing is risk-free. You want to minimize the risk," Eisenberg said.