Both my kids have been exclusively BF to 6 months. DS was BF to 12 months, and DD is only 7 months. DS rarely gets sick. He's been in daycare since 15 months and there's been a slight increase in sickness since then.
DD is sick all the time. She's exposed to many more germy kids than DS was at the same age. And maybe she's just a more sickly person.