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Public health activities are all the ways society coordinates to deliver better health to people. That may sound super broad, and it is, so in this episode of Crash Course Outbreak Science, we'll take a look at public health works to prevent outbreaks. We'll learn what public health is and what public health agencies do, look at the three tiers of public health responses, and revisit one of our favorite fictional cities to see these responses in action.

This episode of Crash Course Outbreak Science was produced by Complexly in partnership with Operation Outbreak and the Sabeti Lab at the Broad Institute of MIT and Harvard—with generous support from the Gordon and Betty Moore Foundation.


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Just as different parts of society coordinate to deliver publicly beneficial goods like roads, clean water, and power grids. Public health activities are the way societies coordinate to deliver better health to people.

Public health takes many forms and interacts with other public bodies like the healthcare system at different levels. It could be networks of microbiology labs that perform clinical testing for a whole country during an outbreak or a single vaccination clinic that boosts the immunity of a given neighborhood against certain diseases. It also includes groups like food standards agencies who make standards to keep our food safe and nutritious, or health and safety lawmakers who create measures to reduce our odds of being hurt in our workplaces but since this is CrashCourse Outbreak Science in this episode we're going to get to know public health in the context of outbreaks, and how it interventions help us overcome them. I'm Pardis Sabeti and this is CrashCourse Outbreak Science
 [Theme Music]
Previously we looked at the role of healthcare systems during an outbreak and how facilities like hospitals treat infected people who become very sick. This makes up the part of outbreak response focused on cures like medicines and treatments which could improve a patient's condition if they're already ill. The other side to tackling outbreaks is prevention which is the focus of public health prevention involves the actions that maintain the health of the population by stopping people becoming infected and getting hospitalized in the first place. The public health strategies we use depend on the outbreak and also who is affected.

As we mentioned in previous episodes outbreaks affect different groups of people differently, depending on factors like our genetics, where we live, our socioeconomic background, even our jobs, and hobbies. These factors are called determinants of health because to an extent they determine our likelihood of catching a certain disease. Determinants are important to take into consideration during a public health response to an outbreak for example ordering people exposed to a disease to quarantine doesn't work as well among people who can't afford to stop working.

But in general we can categorize public health responses into three tiers the first here is primary prevention, which are the actions we take to stop people becoming infected in the first place,these are measures like vaccinating people to make them immune against certain diseases or using personal protective equipment and sanitizing surfaces so pathogens don't infect people. The next tier is secondary prevention where we try to slow the progression of the disease in infected people, that includes things like screenings and testing for diseases that may be slow to develop symptoms like tuberculosis. The final tier is tertiary prevention these are measures that try to improve the quality of life for infected people even if it doesn't directly help stop the disease. Tertiary prevention can include rehab and physical therapy for polio patients and other people with long-term diseases. When it comes to dealing with outbreaks public health responses tend to occur at the tier one level.

To understand it in context let's look at it through the lens of what many of us may think of when we hear the words public health. Public health agencies; these are governmental departments that are responsible for coordinating and implementing public health measures sometimes departments will also have agencies in different regions and cities. In many places like South Korea and Europe they're called the Center for Disease Control though in the English-speaking world the acronym CDC tends to refer specifically to the center for disease control and prevention in the United States.

The tier one preventions that public health agencies implement intersect with all kinds of industries. For example in the U.S.the CDC involves experts in human, animal, and environmental health and other fields to monitor and control public health threats and to figure out how diseases spread among people animals plants and the environment. This approach is called One Health. This approach recognizes the interconnection between people, animals, plants, and their shared environment and many other agencies do the same. For example since three-fifths of all new infectious diseases spread from animals to humans some public health agencies employ veterinarians and are involved in education campaigns to share guidance for handling pets and livestock to limit the potential of a new outbreak.

Other ongoing measures involve monitoring water supplies for the presence of pathogens or working with food standards agencies to create safe practices for food handling and sanitation. For instance in the U.S. one national laboratory network that detects food borne disease outbreaks prevents around 270 000 illnesses every year from the three most common food borne illnesses Salmonella, E.Coli, and Listeria, saving about a half a billion dollars per year in medical costs and lost productivity from people who could have fallen ill . Actions like these are an effective way to keep people healthy and reduce the effect of a potential outbreak to virtually nothing since the outbreak never gets started.

But since, as we've seen in this series, outbreaks do continue to happen public health agencies also play an important role in responding to the ones that do occur. One of the ways they do this is by supporting a discipline we visited in the series already epidemiology. Public health agencies usually host epidemiologists or fund them in research institutions, they also help bring together data from the health care system, field teams, and other sources like community health groups and share them with scientists from many disciplines so they can conduct investigations into the outbreak. This helps us gather the essential facts in the early stages such as what pathogen could be responsible, how it transmits from person to person, or what its reservoir might be; once those are known we're ready to act.

Public health agencies can use the knowledge they've gathered to come up with specific prevention strategies for outbreaks and progress. The exact strategy will depend on the sort of outbreak for example if the outbreak is linked to a contaminated water source public health agencies might work with civil authorities and water suppliers to provide alternative sources of water to stop more people from becoming ill. On the other hand if there are a small outbreak linked to a single infected person entering a country the prevention strategy might involve a combination of contact tracing and quarantining everyone they've come into contact with. In fact public health agencies often play a big role in operating services like contact tracing which are necessary during an outbreak.

At other times a disease might already be spreading rapidly from person to person as in the case of influenza viruses which cause flu outbreaks. When an infectious disease is spreading like this public health agencies can use lots of different interventions to stop an outbreak. To understand how this works it helps to consider cheese, yes cheese. The Swiss cheese model is a way of picturing how to stop a threat from reaching vulnerable people by using different strategies that build on top of one another. It's used in all kinds of places from computer security to aviation but in the context of an outbreak we picture it as a stream of pathogens heading towards people who are susceptible or could become infected. The actions and interventions that prevent a pathogen from reaching them are the barriers that come between them and those barriers are made of cheese. Stick with me.

Like a slice of Swiss cheese our barriers have holes since every public health intervention is imperfect what that means is that a single intervention might not be enough to stop an outbreak. But we can still use these measures to tackle outbreaks because those barriers each have different holes if they don't line up then our stream of pathogens can't get through.  In the language of the real world our multiple interventions can compensate for each other's flaws.

To see how this works we can consider a flu outbreak in the made-up city of Hankopolis, the capital of the fictional country Hankistan. At the start Hankoplus has no measures in place to tackle the flu outbreak in the Swiss cheese model the stream of viruses head straight towards healthy people who could become infected. Their national public health agency the Hankistani Center for Disease Control or HCDC needs to respond it's time to introduce some barriers.

Let's go to the Thought Bubble.

To start with the HCDC encourages everyone with symptoms of the flu to isolate themselves which stops them from infecting others, but some people don't know they're sick leaving a hole in our barrier. So the HCDC also manages and implements contact tracing as people who might have been infected are told to self-isolate fewer people unintentionally spread the virus. This adds a barrier to the model blocking some of the holes from the previous one. But the HCDC can't trace everyone who's exposed to the virus so they also encourage everyone in Hankopoulos to rigorously follow some hygiene measures like covering their mouths when they sneeze or cough, washing their hands with soap and water, and disinfecting surfaces that might have viruses on them. By now much fewer of the viruses are passing through our barriers finally they introduce measures which influence how people interact with one another. By working with workplaces and governments the HCDC sets guidance on the maximum number of people in a group that can gather in a single place to limit the extent to which the virus might spread from a single infected person to many. And that final barrier means the virus begins to spread so slowly that the number of infected people begins to shrink and the outbreak finally begins to fade away.

Thanks Thought Bubble.

In this case the HCDC got things under control with a particular set of interventions. But if the outbreak had been more severe they might have used stricter measures like banning non-essential travel in and out of Hankopolis to stop the outbreak spreading to the rest of the country. Anyway the measures we've considered are only effective if they can be implemented properly. For example people might not self-isolate if they can't afford to miss time from work. Hygiene measures like cleaning surfaces in hospitals and workplaces require supplies of disinfectants which might not be readily available when the outbreak hits. So like we saw for the healthcare system public health's ability to respond to outbreaks depends on the resources that governments, institutions, and individuals have. In fact slowing the spread of the virus through public health interventions is one way to stop the health care system from being overwhelmed.

If we look at the number of people infected over time, interventions make sure that the total number doesn't get higher than the health care system's capacity,we call that flattening the curve. Along with benefits there are also often social costs to implementing certain measures, like restricting the number of people who can meet in a given place and stopping non-essential travel. Disruptions like these to people's lives need to be taken into account to effectively apply a public health intervention we need to manage resources effectively and listen to the voice of the communities affected. When we act quickly and effectively with interventions people can actually follow outbreaks and more easily. But if our interventions are unrealistic costly or simply too late we risk making an outbreak last longer. It all works as a feedback loop and it works best when we're prepared. But reaching a consensus on issues like these and tackling the logistics of interventions is its own challenge and in the case of pandemics it's one that takes global coordination to properly address. We'll take a look at that in our next episode.

We at Crash Course and our partners Operation Outbreak and the Sabeti Lab at the Broad Institute at Mit and Harvard want to acknowledge the Indigenous people native to the land we live and work on and their traditional and ongoing relationship with this land. We encourage you to learn about the history of the place you call home through resources like And by engaging with your local indigenous and aboriginal nations through the websites and resources they provide.
Thanks for watching this episode of CrashCourse Outbreak Science which was produced by Complexly in partnership with Operation Outbreak in the Sabeti Lab at the Broad Institute at Mit and Harvard with generous support from the Gordon and Betty Moore foundation. If you want to help keep CrashCourse free for everyone, forever, you can join our community on Patreon.