Microscopic bugs that you don't see but know they're there. You usually think about them on handles in public toilets or on public transport.
"Ew. Janine just left the toilet but didn't wash her hands. She's just adding more invisabugs to the door handle. Lovely."
Sarah: Does anyone have a fork?
Joe: There's one in my drawer at my desk. It's clean.
Sarah: I'll wash it anyway. God knows how many invisabugs are crawling around in there.