Could you tell me what happens to an untreated case of strep throat?

Complications: Complications can include infection of middle ear, sinuses, tonsils, skin & blood; scarlet fever, inflammation of the kidney (glomerulonephritis) & eventually rheumatic fever.

Lucky vs unlucky: In some lucky ones ( probably the majority) the symptoms subside in 3-7 days and nothing bad happens at all. In some, the germ makes an erythrotoxin that produces a sunburn like rash that causes itching & skin pealing. In some, the body develops antibodies that can cause kidney damage & high blood pressure. In some, the antibodies attack the heart or brain producing rheumatic fever & long term pbs.