In most cases, life of any kind (including your fingers) should be
kept outside of camera equipment. Once in a while, some life forms
find enough food in the camera, and can behave very bad if you do not
take measures. Fungus in lens can be especially dangerous.
Read on.
Fungi in lens. Some people call them molds (moulds
in Britain).
Whatever their classification, they can destroy your expensive lens before
you notice.
Dust mite in lens. Sometimes life in lens can
be useful. An almost unbelievable story told by the eyewitness.