With COVID-19, questions about how infections cause lasting immunity, or don’t, and how you know and what it all means for vaccines have become a matter of public focus. But some immunologists have been pondering those questions for years. “The immune system has a very good memory,” Bali Pulendran told BioWorld. “Clearly, some viruses and some pathogens can enter the body and stimulate the immune system, and the immune system can remember that encounter for decades.”