Well ... Most kids can easily get sick before 3 yrs old!
My friend's kid started going to day-care at 2 months old (it was almost $2000/month 3 yrs ago). Although the ratio was already very low and she kept breastfeeding til 16 months old, her kid got sick almost non-stop in the first 3 years!
Also, think about it - when your kid is sick, s/he won't be able to go to day care so you got to take time off and stay home with him/her. You might wanna make sure your boss can understand and is ok with that.
If you really have no choice but going to work asap, wait til your kid is at least 18 months - 2 yrs old and start going to day care after the flu season.