What is a Patient Zero?

The expression patient zero comes from epidemiology. In medicine it describes the first person infected in an epidemic or a pandemic. 

The expression patient zero comes from epidemiology (the study of the spread of disease.) Epidemiologists are kind of medical detectives. It is important to now where diseases start in order to control them.

The term was first used widely in the 1980s at the start. of the HIV/AIDS outbreak, replacing index patient (or index case), which was in use since the early 1900s.

Patient zero is used in medical situations to describe the first person infected with a disease. Once a patient zero is identified, they can understand better how disease is spreading and work toward a cure.

By the 1990s, Patient zero was being used to describe imaginary scenarios like, say, the first person infected in a zombie apocalypse, taken more generally to mean the origin of a fast-spreading epidemic.

In the case of the Covid-19, or Coronavirus pandemic which started in China, scientists still don’t know who patients zero is. Chinese authorities originally said that the first case was on 31 December and many of the first cases were connected to a seafood and animal market in Wuhan, in the Hubei province.

But then Chinese researchers claimed the first person to be diagnosed with Covid-19, was on 1 December 2019 and that person had “no contact” with the Huanan Seafood Wholesale Market.

The World Health Organization (WHO) has a theory that the outbreak started at the market and could have been transmitted from a living animal to a human host before spreading humans and it is clear to see hw knowing about patient zero would help them to understand more about the virus.

Other Patient zeros in history

The Ebola outbreak in West Africa from 2014 to 2016 was the largest since the virus was first discovered in 1976. More than 11,000 people died and over 28,000 were infected. The outbreak lasted more than two years and spread to 10 countries, mostly in Africa but also in the US, Spain, the United Kingdom and Italy.

Scientists found that this outbreak of a new strain of Ebola started with just one person – a two-year-old boy from Guinea – who may have been infected by playing in a hollow tree that had a colony of bats.

The most well known “patient zero” is Mary Mallon, also known as ‘Typhoid Mary’. She caused an outbreak of typhoid fever in New York in 1906.

Mallon emigrated to the US from Ireland, and worked for rich families as a cook. Anywhere she worked, members of the household started to develop typhoid fever. Doctors called her a healthy carrier – someone infected by a disease but who display little or no symptoms of the disease, which means they often go on to infect many other people.


the branch of medicine dealing with the incidence and prevalence of disease in large populations.


A disease affecting many persons at the same time, and spreading from person to person.


A worldwide outbreak of a new disease, usually a virus.

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