Until the past century, people who survived into young adulthood and beyond we're nevertheless susceptible to death from infectious diseases such as

Until the recent past, which has evidently seen the development of very potent antibiotics, people were susceptible to infectious diseases including influenza, cholera, tuberculosis, HIV, tetanus, SARS, malaria, and skin diseases occurring due to Staphylococcus aureus. 

The answer would be a