Sick again?

How often can a toddler catch a cold? Seriously.

Isaac is sick again, and it seems like it's going to be a bad one. He began sneezing overnight, and his nose started running today -- but already his snot is thick and discolored. We also have poor sleep and crankiness, on both our parts.

Man, he just had a cold two weeks ago, around February 4! It was pretty minor, though. The cold he had six weeks prior to that was much worse, and it lasted from Christmas into early January. He also had colds around November 5, October 11, and September 21. I could go back further, but I think you get the point. (As far as non-cold illnesses, in early December he had an extremely mild case of hand, foot, and mouth disease, but it didn't seem to bother him at all.)

I know we're extremely fortunate that he's never had any serious illness. And as far as annoying garden-variety illnesses, he's never even vomited or had diarrhea. His colds have just been plain old colds -- never ear infections, bronchitis, strep throat, flu, or croup. But still -- should he really be catching a cold every five or six weeks?! I thought breast-fed children were supposed to have strong immune systems.