That's a saying I've heard: "Summer colds are the worst." But you know what? Really they're not so bad. True, this cold does seem to be hanging on for a good, long, annoying while - but for most of its duration, it hasn't been really miserable. I mean, a lot of those winter colds, they can be pretty miserable! This cold, okay, it was pretty bad last couple days, but the preceding two weeks or so, it was alright. It kept pretty reasonably this-side-of-miserable.
So I think it's just a saying. It's just something people say, for those occasions when you ask how somebody is, and they say they have a cold, you just pipe up:
"Summer colds are the worst!"
...and they agree, but really it's just sort of an automatic response deal. Because really, they're not so bad. They're not all that bad.