Sore throat, Scarlet fever, iGAS: What’s the common link?

Strep A infections have been very much in the news this month following the tragic deaths of 15 young children since September. BBC Wales News reports over 18,000 calls to NHS Wales 111 over the second weekend of December, many from parents concerned about Strep A infections.  But what is a Strep A infection and what is the link between sore throats, Scarlet fever and iGAS?