Question: If Frank got Anthrax by eating the egg in the monkey's mouth, shouldn't that monkey have been dead anyway?
Question: What happened to Shane's mom? In one scene we learn that she died because she was sick but what did she have that caused her to die?
Answer: Just like Frank, Shane's mom never ate healthily. Instead, like Frank, she probably just ate whatever she wanted, so she most likely died from a heart attack from eating unhealthy foods or from obesity.
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.