Sickness

Do you have a source? Seems unlikely to be true, unless they're overworking. If anything, cycling makes your immune system stronger.

I fancy that the top pros do a lot of international traveling, so their bodies may not be used to the environments of foreign countries.

These athletes also push themselves to the extreme so it's harder to take care of their bodies appropriately. Also, during long stage races you can't get adequate rest if something goes wrong. Normal people would rest if they had a slight stomach ache, but racers who continue to race through the pain can make things worse.

Racing is also not very sanitary. You don't get to stop and wash your hands while racing and you can't really do the things that you would normally do to make sure the things you drink and eat are clean, etc.

serra said:

Do you have a source? Seems unlikely to be true, unless they're overworking. If anything, cycling makes your immune system stronger.

I was just thinking that because cyclists have such low body fat percentages (which if your body fat percentage is too low it can comprimise one's immune system) along with their extreme training loads that they might have slightly compromised immune systems.

This is not fact. It just just an observation and an opinion from a non-expert.