Because cleanliness was not a major priority in the lives of our ancestors since the dawn of time, diseases, especially those caused by bacteria, were a common occurrence in their daily lives.

Major killers listed are no longer killers today. Infectious diseases are still a common scourge among third-world countries where sanitation is a challenge and health services are not that accessible. However, we have not yet conquered all infectious diseases, and there is still the danger of drug-resistant strains of bacteria.