Our proposed study will measure the effect of an oral probiotic, Strep salivarius (a healthy naturally occurring bacteria that is found normally in the mouths of some children) to prevent Strep sore throats in a population at high risk of Rheumatic Fever. Children who naturally have these bacteria in their mouths have less Strep sore throats. The Ministry of Health sore throat treatment programme currently running in East Porirua schools, in children aged 5-13 years, provides a unique opportunity to test whether this natural product currently available in pharmacies (and made in NZ) can reduce Step sore throats. A small pilot study has shown a 90% reduction in Strep sore throats in high risk populations but no formal blinded randomised trial has previously been undertaken. If effective, this preventative strategy could be incorporated immediately into the prevention of Rheumatic fever (by preventing Strep sore throats) in all high risk populations in NZ.