Other mistake: When Drix agrees to stay on the case with Osmosis, Ozzy looks up in surprise. In the next shot, Leah says "wow", then it cuts to the very same shot beforehand where Osmosis is surprised again. (00:40:35)
Other mistake: During the scene where Thrax is talking to the other germs and viruses in the Zit and is pointing out things on the map of Frank, the word 'collarbone' is spelled wrong ('Collerbone'). (00:52:49)
Other mistake: While Osmosis is stranded on the false eyelash, he turns around and screams before Thrax even made his presence known behind him. His attack and Osmosis' response to it is completely out of sync. (01:21:00)
Other mistake: After Jones is fired and walking the streets and the "Daily Frank" are tossed out, you see the front page. Under his picture it says "Osmosis Jones in shock." But under that it's just gibberish.
Answer: We are never told that the monkey was sick with the disease. The egg does fall out of the monkeys mouth and onto the ground where the virus could have easily been on the ground and it just happened to come into contact with the egg.