Public health and epidemics

Medicine often focuses on treating individual patients. Public health focuses on preventing disease and promoting health across entire populations, through measures such as clean water, sanitation, vaccination, food safety, health education and laws on tobacco and seat belts.

Its achievements are enormous. Global life expectancy has roughly doubled since 1900, rising to over 70 years. Much of that gain came not from hospital treatments but from preventing infectious disease, especially among children.

In 1854 a cholera outbreak killed hundreds of people in the Soho district of London. At the time, many doctors believed cholera spread through bad air, called miasma. The physician John Snow suspected contaminated water.

Snow mapped the deaths and found they clustered around a water pump on Broad Street. He persuaded local officials to remove the pump's handle, and his investigation became a founding example of epidemiology, the study of how diseases spread in populations. Later work confirmed that cholera is caused by bacteria spread through water contaminated with sewage.

• Sewer systems, such as Joseph Bazalgette's network built in London after the Great Stink of 1858, separated sewage from drinking water.
• Water filtration and chlorination sharply reduced typhoid and cholera deaths in the early twentieth century.
• Germ theory, established by Louis Pasteur, Robert Koch and others, explained that specific microbes cause specific diseases.
• Pasteurisation of milk prevented diseases such as tuberculosis spreading through dairy.
• Handwashing, promoted by Ignaz Semmelweis in maternity wards in the 1840s, reduced deaths from infection.

An epidemic is a rapid rise in cases of a disease in a region; a pandemic is an epidemic that spreads across countries and continents. How fast a disease spreads is often described by its reproduction number, R: the average number of people each infected person infects.

If R is above 1, cases grow; below 1, the outbreak shrinks. R depends on the pathogen, how it is transmitted, how long people are infectious, how many contacts they have and how many people are immune. Measures such as vaccination, isolation of the sick, ventilation and masks work by pushing R below 1.

The Black Death of the fourteenth century, caused by plague bacteria, killed perhaps a third or more of Europe's population. The influenza pandemic of 1918 to 1920 infected about a third of the world's population and killed tens of millions of people.

HIV, first identified in the early 1980s, has caused tens of millions of deaths, though antiretroviral treatment now allows people with HIV to live long lives. The COVID-19 pandemic, declared by the World Health Organization in March 2020, caused millions of deaths worldwide and showed both the power of rapid vaccine development and the importance of preparedness.

Vaccination is one of the most effective public health tools ever developed. When enough people are immune, a disease struggles to find new hosts, protecting those who cannot be vaccinated, such as newborns or people with weakened immune systems. This is called herd immunity.

A global vaccination campaign led to the eradication of smallpox, the only human disease so far wiped out; the World Health Assembly declared it eradicated in 1980. Polio cases have fallen by more than 99 percent since 1988. Today's public health challenges also include non-communicable diseases such as heart disease and diabetes, mental health, antimicrobial resistance and the health effects of climate change.