I don't understand this. Before I had my daughter, I'd been sick twice in my life. Now I get sick every month?

Ok so maybe that's a *little* over-exaggerating, but it's pretty darn close. Before I had my daughter, all throughout childhood and being a teenager, I had a gold medal immune system, didn't get sick not once but every three or four years. Now though, ever since I had her when I was 20, I get sick with colds and flu 4-7 times *every* year. Did she really take that much of my immune system? Or...is there something else going on that would make me go from super immunity to one sick schmuck? =P What gives?