Up until the early-to-mid 1900s — when sanitation and water quality improved, living conditions were enhanced, and antibiotics and vaccines were developed — infectious diseases were by far the leading cause of death worldwide. The worst of them was Tuberculosis, widely considered the deadliest infectious disease in human history. Over the centuries, this infectious disease has ravaged humans, killing around one in seven people who have ever lived. Looking at the past two centuries alone, Tuberculosis has killed more than one billion people. Cures for Tuberculosis were finally developed in the 1940s and 1950s, but it still managed to kill more than a million people in 2023 — mostly in poor, malnourished developing nations where medicine is either scarce or unaffordable. 

So, what is this deadly disease? Tuberculosis is an airborne bacterial infection caused by the bacterium Mycobacterium Tuberculosis. It spreads through small particles in coughs, sneezes, and exhalations, and it is incredibly common. In fact, between one-quarter and one-third of the world’s population carry Tuberculosis. In most people, the infection will lie dormant for a lifetime, never causing problems. But up to 10% of infected people will become sick — when this happens, it’s called Active Tuberculosis.

Once active, how does Tuberculosis do its killing? It essentially “chokes” patients by attacking and growing in the lungs, causing inflammation in the area so severe that patients feel like they’re breathing through a straw or drowning. It also causes severe coughing, a lack of appetite, and stomach pain — all of which result in patients losing significant weight and sort of “wasting away” until they die. These horrible symptoms are why the disease was called “Consumption” until the 20th century — it literally consumes the body from within. As the author writes: “The afflicted often drown as blood and pus fills their lungs. They die starved of oxygen, desperate for air.”

Slowness is the signature of Tuberculosis. Compared to other infectious diseases — like E. Coli — it has an uncommonly slow growth rate. This is because the Mycobacterium Tuberculosis bacterium needs a lot of time to build its unusually fatty, thick cell wall. The white blood cells of our immune system struggle to penetrate this thick wall and kill the bacteria inside. Instead, they choose to surround the bacterium, creating what is called a Tubercle. 

The Tuberculosis bacterium can survive inside of these Tubercles for a very long time, which is why it often lies dormant inside somebody for a lifetime. The Tubercles basically prevent the bacterium inside from becoming active. But in 5-10% of cases, the immune system can’t create enough white blood cells to surround the bacteria and create Tubercles, allowing the Mycobacterium Tuberculosis bacterium grow in the lungs and cause inflammation in the area. The body is then slowly overwhelmed until the lungs collapse or fill with fluid, leading to death.

What causes Tuberculosis to become active? In short, a weak immune system. The cures developed in the 1940s and 1950s basically put an end to Tuberculosis in rich countries, but poor nations still struggle mightily with it since the disease is more likely to become active in people living in places where there is widespread malnutrition and other factors that cause the immune system to be compromised. A weak immune system allows Tuberculosis to break free from the Tubercles and spread through the lungs, causing severe inflammation in the area. Treatment is vital to survival — without it, about half of active Tuberculosis patients die within five years.