I once walked through an old graveyard near Philadelphia and noticed the years of births and deaths carved on the headstones. It reminded me that up until the early 1900s, most people died before their 50th birthday. The primary causes of these deaths were infectious diseases such as smallpox, influenza and pneumonia.
Today contagious illnesses kill more rarely in developed nations, where improvements in sanitation, nutrition and vaccines and the introduction of antibiotics have virtually eliminated premature deaths from such afflictions. Yet we are perilously close to returning to an era of untimely deaths from these illnesses because many microorganisms are becoming resistant to existing drugs and because the pharmaceutical industry is not developing enough replacements.