Rheumatic fever is a strange disease. It begins as something ordinary — an infection of the throat caused by a common bacterium called group A Streptococcus, the same organism behind ordinary strep throat. In the vast majority of cases, the throat infection clears, with or without treatment, and the patient recovers entirely.

In a small fraction of cases, something else happens. Two to four weeks after the throat infection has cleared, the immune system, which has been producing antibodies against a particular protein on the bacterial cell wall, continues attacking. The problem is that a similar protein appears in the body's own connective tissues. The immune system stops being able to distinguish self from invader.

The result is acute rheumatic fever. The misdirected immune response inflames the joints, sometimes the brain, the skin, and most damagingly, the valves of the heart. Repeated episodes leave the heart valves permanently scarred. The long-term condition, called rheumatic heart disease, can cause heart failure decades later.

For most of human history, rheumatic fever was one of the leading causes of childhood heart disease in the developed world. The composer Gustav Mahler is believed to have died of complications from heart valve damage caused by rheumatic fever in his youth.

The intervention turned out to be unglamorous and powerful. A short course of antibiotics for the original strep throat prevents the immune confusion from beginning at all. In countries with reliable access to penicillin and pediatric care, rheumatic fever has nearly vanished.

In countries without that access, the disease continues. The World Health Organization estimates that around 30 million people worldwide live with rheumatic heart disease today, and that the condition kills hundreds of thousands every year. A simple antibiotic course is enough to prevent it.