Two types of bacteria cause impetigo: streptococcus pyogenes (strep) and staphylococcus aureus (staph). The condition is highly contagious.
In preschool-age children, the sores mostly appear under the nose and around the mouth. In general this is not a serious illness, but a passing infection which often clears up on its own within three days. In more serious cases, impetigo is treatable with antibiotics.
Since small children are still developing immunities to infection, they are more likely than older children and adults to contract the infections which may lead to impetigo. Similarly, older children and adults with compromised immune systems would also be at a higher risk for contracting it and certainly for passing it along since it doesn't discriminate when it comes to infecting others.
While many times impetigo is treated with oral antibiotics, it is also common to treat this condition with topical antibiotics, with favorable results.