I have heard the same as Em-aiden about kids being sick when they were immunised and the link to autism. The nurses always ask if your baby is sick, on medication etc...even if they have a little cough, or slightly different colour poo, tell the nurse.
When they immunise, they put a little of the virus in to immunise. But if they are sick, immunity is low so that virus may do more harm. In a healthy baby, the natural antibodies work against the virus to stop them getting sick from it and doing them any harm.
This is just simplified and what I got out of the readings. If your baby is healthy, it should be fine. If they are not immunised what could happen? Immunisation is such a debated topic. I personally would prefer to immunise against all those nasty diseases which can sometimes paralyse a child then to not immunise them. But only when they are healthy to start with!