My first son was 4 weeks early. Luckily, he was born healthy, just ended up with severe juandice. I only had 7 weeks off. I found a small in-home provider close to work that was state certified and inspected for preemies. The place was less than 10 min from work, so I'd go and nurse him during lunch. I had easy work hours, too, so that helped. He was born in Feb, started daycare in April, so I think we lucked out by missing the sickest time of year. We moved, and ended up in a very small center.
That said, it seemed he was still sick a lot. Lots of ear infections, ear tubes put in on his 1st birthday. No ear infections since. The first two years we also had a lot of stomach bugs.
With DS2, we had moved, and moved again, and were (and are) now in a large center. DS2 was born healthy at 39 weeks, and I was able to stay home 12 weeks. I started dropping him off at daycare for a few hours a few days a week after he was 2 months old - so I could workout or shop or just have mommy time. He started full time daycare in Dec. He has been sick about the same as DS1 - lots of ear infections (tubes scheduled for Aug), and lots of colds that bring on breathing issues. But he also had an unexplained illness when he was just a few weeks old and not in daycare. He also got exposed to chicken pox last week (only 9 months old so not vaccinated for chicken pox yet) and were waiting to see if he gets chicken pox.
IMHO, smaller centers mean less germs. I wish we were in a smaller center, but the one were at now is by work and I get a huge discount through my employer. Its also state of the art and less than 2 years old.
Good luck on finding the right daycare for you!