Nasty red spots on the face and around the bite area. Go nutty,make a hole (seemingly) in the skin, glue some small plastic spiders around the hole (think "Volcano") "Are they being born inside this victim's flesh? Yummy blood to feast upon, you know!"
The more advanced Victim could be sporting some small spider arms popping out from their arm pits.
"But I Can't be a Spider! I'm not smart enough to move all those legs right!"
Then they fall almost bumping into the next potential casualty.
Pull an empty dozen chicken egg container out to look at the "Babies!"
Small plastic spiders glued to threads, dangling from the egg carton would be nice.