I'm having this endless dispute with my mother: she cleans her computer keyboard everyday, is careful not to put her handbag on the table etc... in the name of keeping bacteria away. This kinda makes sense, since bacteria can cause illnesses, but... I don't see any reason why it couldn't be the other way, meaning that more exposure to common bacteria could lead to less illnesses. After all, immune system learns from the bacteria entering the body and therefore could be better prepared for illness if it saw the same bacteria before(but in smaller numbers). Does anyone know which way it is?