Hmm... what kind of sickness? Fever? Vomitting?
If it's just a cold and LO doesn't have a fever, I would think it would be OK. However, if a fever is involved, I would politely decline. I know someone who just had a baby this week and the baby had to be put on antibiotics when he was born b/c the mom was running a fever and was ill (not that antibiotics are bad, just saying illness could cause complications in a number of ways).