Infectious diseases can strike at any time. Some of them cause relatively minor interruptions to daily life and often can resolve of their own accord when the body’s immune system mounts a successful defense. Other diseases can cause serious, even life-threatening, symptoms or spread rapidly, which makes it essential for medical professionals to help slow down or stop the transmission.

Herd immunity refers to the indirect protection from infectious diseases that occurs when a large percentage of the population has become immune to that disease. The term has taken on renewed significance as the world has been battling COVID-19.

If enough people are resistant to the cause of a disease, whether it is a bacteria or virus, that disease has nowhere to go and the spread stalls, according to WebMD.

There are two ways that herd immunity can occur. The first is when resistance develops naturally when the body is exposed to the virus or bacteria. At this point, the immune system will produce antibodies to fight off the infection. After recovery, these antibodies are still circulating, and should exposure to the same disease occur again, the body can defend against another infection.

Another way that herd immunity occurs is through vaccination. The Centers for Disease Control and Prevention states that when the majority of people are vaccinated, it creates the same disease lockdown, fewer people get sick and fewer germs are able to spread from person to person.

Diseases are different and herd immunity is reached based on the pathogen’s reproduction number, or R0 (R-naught).

The R0 tells the average number of people that a single person with the virus can infect if those people aren’t already immune.

The higher the R0, the greater number of people will need to be resistant to reach herd immunity. Measles, which is very contagious at an R0 of 12 to 18, requires 93 to 95% of the population to be immune for herd immunity to be reached.

The World Health Organization estimates the R0 for COVID-19 to be between two and three. This means between 40 and 70% of the population will need to be immune to halt the spread.

In the case of COVID-19, it’s still unclear whether anyone can get re-infected, and whether antibodies produced for one strain can fend off another strain of this novel coronavirus.

This reinfection mystery is what makes herd immunity, both through a vaccine or through natural exposure, challenging for epidemiologists in relation to COVID-19.