in 2014 an outbreak of the deadly virus ebola was spreading across guinea a country in west africa the outbreak had started in a small village but spread rapidly by the time health officials were able to end the outbreak over two years later there had been more than 11 000 deaths associated with the virus the ebola outbreak is a classic example of an epidemic when more people in a group than usual develop a particular illness or condition but epidemics don't need to be an apocalyptic viral event that dominates headlines for example as of 2019 over 80 percent of school-aged children in china singapore and south korea were nearsighted which is more people in that group than we would expect to have that condition so we could say that near-sightedness is an epidemic but no matter which kind of epidemic we're studying we need epidemiology to help us do it epidemiology is basically where all the sciency stuff happens in public health it uses microscopes data math and pie don't worry we'll get to that hi i'm vanessa hill and this is crash course public health [Music] epidemiology is the study of the patterns of disease and health conditions within populations it also studies the patterns causes and how they can be controlled much like democracy and the mamma mia film franchise we can at least partially give credit for the word epidemiology to the greeks it comes from the three greek words epi demos and logos which mean upon people and study so epidemiology is literally the study of what is upon the people which is kind of a terrifyingly broad field like what is upon the people could describe anything from climate change to whatever hank green is plugging on tiktok today be sure to check out the link in his bio but anyway to specify an epidemiologist or someone who studies epidemiology wants to know who gets what diseases where they get them and when epidemiologists are kind of like detectives in a massive game of clue or cluedo except instead of professor plum in the ballroom with the candlestick it's more like a bacterial infection on planet earth that threatens literally everyone the word and the actual practice of epidemiology didn't gain traction until the 19th century when it mostly concerned infectious diseases these are health conditions caused by organisms like viruses bacteria and parasites which are spread between people or picked up from the environment or animals but today we understand epidemiology more broadly this is partly because we also understand health more broadly and as we saw in our episodes on the determinants of health our health is affected by more than just germs it's also impacted by our neighborhoods schools and society in general but we also understand epidemiology differently because at least in high-income countries causes of death have changed as advances in medicine and public health have meant fewer deaths from infectious diseases in these places non-infectious also known as non-communicable causes of death have become a bigger area of focus we know that's pretty unfair for lower income countries we'll talk about that in episode 9 when we focus on global health so today many epidemiologists put more emphasis on studies of non-communicable diseases like cancer heart disease and diabetes environmental factors like air pollution and even the health impacts of natural disasters they also examine the determinants of health and the inequities in who gets sick that's also what we'll be focusing on today though over at crash course outbreak science we spend plenty of time talking about infectious diseases including how public health tackles them we know with any big mystery it's not enough to know that something happened we want a motive so as epidemiologists solving a health mystery we want a cause for that health outcome a health outcome is what happens basically any time our health status changes because of well something happening in the world this it could be a good outcome like having lower cancer risk thanks to air pollution laws or a bad outcome like having higher cancer risk thanks to factors like only being able to afford to live in an area close to a chemical plant epidemiologists begin with a hypothesis about why a health outcome is spreading or occurring in the first place then they conduct a scientific study to evaluate their hypothesis in general there are two kinds of epidemiological studies that we'll focus on here experimental studies and observational studies in an experimental study investigators expose participants to some kind of intervention or treatment to see how it affects their health then they compare the outcomes to a control group that isn't exposed to the interventional treatment now it's pretty unethical to expose a group of people to something that could harm their health so experimental studies tend to introduce positive interventions this might include something like a new vaccine as opposed to a negative intervention like a virus in observational studies epidemiologists observe a population that is already exposed to a particular treatment or risk factor and compare their health to a non-exposed group this is how we go about understanding the effects of things we don't want to intentionally expose people to like viruses one famous observational study was conducted by a pair of british epidemiologists richard dull and austin bradford hill no relation to me in the mid 20th century it was widely known that lung cancer rates were on the rise but there wasn't a scientific consensus on why in 1951 dull and bradford hill began testing a hypothesis for this increase smoking the idea that smoking can lead to certain kinds of cancer is pretty common knowledge today but while some research pointed to an association between smoking and cancer there wasn't enough evidence to confirm that one event caused the other in fact smoking wasn't even formally recognized as a public health issue in the united states at the time they sent out surveys to almost 60 000 british doctors asking about their smoking status in a study that experts have so ingeniously dubbed the british doctor's study and after getting over 40 000 responses dull and bradford hill found a strong association between heavy smoking and lung cancer and repeated follow-up studies of the same doctors over the next 50 years confirmed the originally reported relationship of smoking to several different kinds of cancers including lung and mouth cancer unfortunately when it comes to identifying the cause of a particular health effect we don't always have the luxury of 50 years of research and 40 000 british doctors on hand epidemiologists often have to work quickly to assess and respond to a health emergency regardless of the timeline involved interpreting data is where epidemiology gets tricky because data on its own don't tell a story for data to be useful they need to be interpreted by people and data aren't always straightforward like data have shown that there is a near perfect relationship between a population's level of cheese consumption and the number of people in that population who die from getting tangled in their bed sheets this is a very strange but true example of a cliche that scientists have been muttering in their sleep for centuries correlation doesn't imply causation or put another way just because two things seem related it doesn't mean that one caused the other so while the data appear to say that cheese consumption and bed sheet murder both increase at the same time we need someone like an epidemiologist to examine and interpret the data to tell us whether that actually means anything or if cheese consumption just happens at a similar rate to a lot of other things to successfully interpret data epidemiologists need to rely on several pieces of evidence like while the british doctor study was one of the first population level studies that linked smoking to lung cancer this wasn't a new idea at the time as dull and bradford hill conducted their studies they were following in the footsteps of animal-based studies and chemical analyses of tobacco that had been happening as far back as the 1900s like a lot of science their conclusion wasn't the product of two dudes doing a thing it took decades of collaboration between a bunch of people in different disciplines each following their own unique path of evidence to the same conclusion one useful tool epidemiologists use to understand the cause of an observed effect is the bradford hill criteria and yes that's the same bradford hill from the british doctor study small world while we won't get into all of them here bradford hill proposed nine principles for establishing evidence of a causal relationship between a presumed cause and an observed effect like whether the effect happened after the cause or if the effects could be reproduced another tool epidemiologists use to better understand the sometimes messy relationship between cause and effect is mathematical models these models tell epidemiologists which variables are worth paying attention to and which ones aren't which brings us at long last to pie the rothman causal pie is a model that helps epidemiologists explain how individual risk factors contribute to a disease and like any good pie a causal pie needs a few different ingredients except instead of sugar and rhubarb this pie is made up of component causes which probably settles the debate about the worst pie flavor ever component causes are basically the different risk factors that work together to produce a certain health effect pi if we have enough cause slices to form a whole pie we have a sufficient cause which means the health condition goes into effect let's go to the thought bubble let's consider tuberculosis or tb a highly infectious disease caused by the bacteria mycobacterium tuberculosis because tb is an airborne pathogen it's generally spread through a pretty basic human action breathing and when you have too many people breathing in the same space and one of them is sick there's a higher risk for transmission so we might consider overcrowded homes and communities to be one slice of the causal pie similarly buildings with poor ventilation will be worse at getting rid of airborne bacteria making poor ventilation another notable component cause another component of our pie is having a weakened or compromised immune system basically if our body isn't as good at fighting off disease it's also more likely to get sick from exposure to the bacterium similarly lack of access to a tuberculosis vaccine means our bodies will be more likely to contract the disease so that's also a pretty big part of the causal pie just like how everyone cuts a pie differently everyone will have a different combination of component courses that combine to produce a sufficient cause for tb but one slice that's in every causal tuberculosis pie is exposure to mycobacterium tuberculosis this is because tb can't like spontaneously generate in our bodies it needs a source this is why exposure to mycobacterium tuberculosis is a necessary condition of tb even if all the other risk factors of the pie are in place mycobacterium tuberculosis is needed to activate the disease now while the presence of each individual risk factor increases the likelihood of tb it doesn't guarantee it but if we fill up our causal pie with a combination of components that produces a sufficient cause we get our health outcome tuberculosis thanks thought bubble not every disease has an easily identifiable necessary condition like tb does like the causal pies for high blood pressure might not necessarily share a slice in common instead enough component causes can combine in different ways to form a sufficient cause and this enoughness varies from person to person now disease models aren't completely human error proof they're still invented by people and rely on data collected by people but the more data we collect and interpret the better our health models get and in a big messy world where the causes of disease are often invisible where cheese and murderous bed sheets go hand in hand epidemiology gives us the tools we need to make a little bit more sense of the world and how it impacts our health we'll continue our examination of this big messy world next time when we talk about health systems see you then thanks for watching this episode of crash course public health which was produced by complexly in partnership with the american public health association if you want to learn even more about public health head over to apha's youtube channel to watch that's public health a series created by apha and complexly crash course was filmed in the castle garrity studio in indianapolis indiana and made with the help of all of these smart people if you'd like to help keep crash course free for everyone forever please consider joining our community of supporters on patreon