While the majority of cases are relatively mild, sometimes strep can cause a life-threatening invasive Group A Streptococcal infection (iGAS), when it gets through the body’s defences and passes into areas it is not normally found, such as in the blood, lungs or muscles. Two of the most severe but rare forms of iGAS are necrotising fasciitis, also known as the “flesh-eating disease”, and streptococcal toxic shock syndrome which can cause low blood pressure and damage to organs.