I had untreated strep when I was a kid, because the strep screen came back negative. Turns out it was group C, which didn't show up on the strep screen, but also didn't resolve itself. Some months later it got much worse. Somebody finally did a culture, figured it out, and gave me antibiotics (which cleared it up). Fast forward to age 29, when I had to have open heart surgery to get a mechanical mitral valve, for which I have to be on warfarin for the rest of my life (which comes packaged with increased risk of stroke and hemorrhage for the rest of my life) along with a beta blocker ('bystolic', aka nebivolol) because something happened to mess up my heart rate regulation during the surgery. I'd call strep pretty serious.