Understanding an epidemic is like solving a puzzle or doing detective work.

The year is 1981. A team of experts gathers at the Centers for Disease Control and Prevention in Atlanta, also known as the CDC. Together, they’re trying to solve a mystery: Why are young, healthy people in New York and California suddenly experiencing rare diseases like Pneumocystis carinii and Kaposi’s sarcoma? 

Of course, now we know the answer. These individuals were among the earliest known victims of HIV. However, at the time, their conditions were an enigma. In order to solve the mystery, the CDC needed to gather more information about the case. They needed an epidemiologist, and Dr. Michael Osterholm was there to help.

The key message here is: Understanding an epidemic is like solving a puzzle or doing detective work.

The goal of epidemiology is to track and trace the spread of diseases so that they may be controlled and prevented. To do this, epidemic specialists must collect as much information as possible about a given case. Who is contracting an illness? Where is it appearing? Are there patterns to be found?

That’s why, back in the early 80s, the first step that Osterholm and the CDC took was something called “case surveillance.” This involved surveying doctors in New York and LA to find similar cases of people presenting with rare diseases. They found a pattern: the victims were most often young, gay men. The men were experiencing conditions that usually only appear in much older individuals.

As the CDC gathered more information, it became possible to start describing the culprit. This is called “case definition.” In this case, the facts emerged quickly. The disease was a retrovirus, one that attacks the immune system. It is spread by blood transfusions and sexual activity. It most likely originated in sub-Saharan Africa. And, most importantly, it was completely new.

For Osterholm, the HIV outbreak was a Black Swan event. This is the term used by epidemiologists to describe unusual occurrences that have a huge impact. And the impact of HIV has been enormous. Within mere decades, the disease has gone from a few hundred cases to infecting an estimated 40 million people. It is no longer a pandemic, but a hyperendemic– a public health problem that won’t go away.

In this case, the work of the CDC’s epidemiologists couldn't overcome the spread of the disease. It did, however, lay the groundwork for future research into prevention and cures. In the next blink, we’ll hear about another case that had better results.