ALT So what is this ‘immunity debt’ concept? Immunity debt is a newly coined term. The theory claims that lack of exposure to infections weakened children's immune systems, causing them to have severe infections. There is no evidence that this is real. In the absence of infections, the immune system does not lie dormant and weaken. We are constantly exposed to, and co-exist with, trillions of viruses, bacteria, and fungi. The immune system constantly adapts to these exposures. Lack of infections does not weaken the immune system. But some infections, like measles and COVID-19, can.