“I have never heard of such an infection causing infections in other parts of the body”
Oh yeah, left untreated strep can severely damage your kidneys. It has been discussed by forensic medical specialists who resd through journals that given the short time span and the onset of the illness - Mozart may have died from a strep infection.
When I was young I would get strep throat every year it seemed. The doc would swab my throat and then put me on whatever medication (probably a antibacterial) and I would be fixed up. I think in third world countries it’s very serious concern but ever since the advent of penicillin and other drugs that can take care of the strep infection it hasn’t really been an issue. But it is a very serious illness especially if left untreated.
Do you suspect the parents of the 6-year-old may have neglected treatment, so it got worse? From what you wrote, it looks like medical treatment was neglected. Maybe the coughing was not bad enough to take action. It may have been a fast acting form of the infection not ordinarily found in this country—something that came in elsewhere.