Taking a closer look at immune 'memory' could spur progress in the fight against lethal illnesses

The average human has about 1.8 trillion immune cells. These cells patrol the body for bacteria, viruses, cancers, and other threats. Vaccines enhance this security system by teaching our immune cells to target specific pathogens. According to the World Health Organization, vaccine-induced immunity saves about six lives every minute. But how long does this protective immune "memory" last?