How do scientists study new viruses? Epidemiology

If there is anything to be thankful for about the COVID-19 outbreak, it’s that it is happening at a time when scientists have more tools than ever available to study new diseases. Doctors and scientists studying the virus have already made discoveries that even as recently as 15 – 20 years ago would probably have taken several years. Chloe, who is 9, has asked me to explain how scientists study new viruses. Four of the most important methods are epidemiology, genome sequencing, microscopy and crystallography, and in this post I will explain about epidemiology.

Victorian cartoon showing the conditions in big cities that allowed cholera to spread.

Epidemiology is the science of how a disease spreads. It may not sound that interesting but it’s one of the oldest methods of studying disease. The science of epidemiology started in 1854, and was the result of an outbreak of cholera. Cholera is an infection of the intestines that is caused by bacteria. It causes severe diarhea and vomiting, and left untreated it frequently results in death. Cholera bacteria are spread when faeces (as in poo) from infected people get into a water supply, and other people drink the water.

John Snow, the doctor who investigated how cholera spreads.

In the 1850s, cholera was a major problem in big cities because of overcrowding and poor living conditions. Poor people were crowded together in slums and did not have clean water or proper sewage systems – ideal conditions for cholera to spread. In 1854 there was a large outbreak of cholera in London, and it was centred on Broad Street in Soho, and the surrounding areas. A doctor named John Snow used a map to mark where cases were located, and noticed that they were all occurring in houses that used the same water pump, located in Broad Street. John Snow realised that cholera was being spread by water from the pump, and had the handle removed.

Dr John Snow’s map of the 1854 cholera outbreak. Houses were there was cholera are shown in black, and the Broad Street pump is circled in red.
The Broad Street pump, with the handle removed. It’s actually a replica.

Snow’s investigations did not stop there. He started looking at cholera cases for the whole of London, and realised that the majority were occuring in areas supplied by two water companies, both taking their water from the River Thames and supplying it to households without any attempt to clean it. Parts of London that were supplied by companies that got their water from clean sources, and filtered it before supplying it to households, had far fewer cases of cholera.

In 1858, engineer Joseph Bazalgette began work on a system of sewers and pumping stations to prevent sewage from running through the streets or being dumped in cesspools underneath houses. As a result, cholera was completely eliminated from London’s water system. The spread of other diseases such as thyphus and thyphoid decreased dramatically.

So, how will will scientists be using epidemiology to study COVID-19? In the first few days and weeks after the virus appeared, scientists in China used epidemiology to investigate whether the victims had anything in common. Just as John Snow was able to link the 1854 cholera outbreak to the Broad Street pump, Chinese epidemiologists were able to link COVID-19 to a livestock market in Wuhan. This led them to realise that the virus had originated in animals sold at the market, which is really important information for other scientists studying it.

Map showing cases of COVID-19 in China and surrounding countries on January 23rd 2020

Once the virus started spreading more widely, epidemiologists will be looking at where the virus appears and how quickly it spreads. Epidemiologists working for organisations like the World Health Organisation (WHO) are constantly monitoring the spread of the virus. This provides important information about how the virus is spreading. It also allows scientists to evaluate how well measures to prevent or slow down the spread are working; they can then use this information to advise governments on what they should do to try and slow down the spread of the virus in their own country.

I hope this has been useful. If you want to see the latest map from the WHO, you can find it by following this link: