The Next Paradigm Shift in How We Think About Causality

If you paid attention in history class, you may remember that there was a time when we didn’t really know what caused diseases in humans and animals. We used to think that diseases were curses from the gods or the actions of demons and spirits. Because disease was so mysterious, we gave it a fantastical origin. Then came the scientists.

In 1854, an outbreak of cholera was taking a heavy toll on London. Dr. John Snow decided that he was going to ask where and when people were coming down with cholera. He set about to conduct a survey of the city, coming up with the well-known “Snow Cholera Map.” From that, he theorized, correctly, that cholera was not an infection caused by miasma (bad air). He theorized that it was due to contaminated water. What it was contaminated with would take a while to be figured out, but John Snow knew it was the water and did something about it. This was the era of “hygiene” with regards to public health and to causation of disease: bad hygiene led to bad outcomes.

It wouldn’t be until the late 1800’s that scientists like Louis Pasteur and Robert Koch figured out that bacteria (and yet-to-be-discovered viruses) caused disease. We know Pasteur from his work with rabies and with protecting milk from spoiling early. Robert Koch figured out transmissibility of disease through microbes, finally giving the cause of cholera as being a bacteria. That is, they built on the work of John Snow and others to develop “Germ Theory.” This was the era of infectious disease.

Once infectious diseases were under control, people started living longer and better lives. Populations grew older, and, because people were better off, more and more of them ate more and more calorie-rich and fatty foods. Also, more people meant a greater workforce and more demand for goods and services. The technological advances of the 1900s, together with a greater population and growing countries in the Americas and Asia, led to the industrial revolution. The industrial revolution brought polluted air and water, while bigger cities and factories brought injuries. This was the era of chronic disease and public health, an era that lives through to today.

With over 7 billion of us on the planet, and with global climate change manifesting itself more and more, we’ve entered what some term “ecoepidemiology,” where we look at everything from our genes to our planet as the cause of disease. Some people have genes for cancer, but they also need to be exposed to certain environmental triggers to get the cancer. If they live in a well-off area, they may never get exposed. So you can see how genes, social, and financial factors all come into play when it comes to something like cancer. The same can be said of HIV/AIDS. Initially, the epidemiology pointed to homosexual men as the only population affected. Thus, the only risk factor seemed to be sexual orientation and gender (and maybe geographic location). But the epidemiology changed. Everything changes.

I was sitting in a lecture on Friday when the lecturer asked what we saw as the future of epidemiology. I told him that I saw the people (laypeople, people off the streets) being more and more involved and knowledgeable about epidemiology. That has been my intent all along since I became an epidemiologist, to explain Epi to the masses. A good example is “Google Flu Trends” or “Flu Near You,” where the people give the data and warn us to what’s going on in the community. The lecturer mentioned that the MMR-Autism hoax is a good example on why everyone needs to understand epidemiology.

However, another person in the audience, a professor, told us that the needs to be a “paradigm shift” when it comes to how we look at causation. It used to be we thought it was the gods that caused disease. Then we figured out it was about hygiene. (Dirty water meant intestinal disease.) Later, it was the microbes. And then it was the environment and aging. When we figured out DNA, it was the genes. (Just look at how many companies want to sell you your own DNA profile to “predict” your risk of diseases.) And now we’re trying to put it all together. However, the professor said we need to stop thinking of causation as a “Newtonian model.”

If you remember history, you remember that Newton figured out that things in the universe behaved in pretty much a predictable way. What goes up must come down, etcetera. We can figure out and predict the position of the planets based on what Newton told us about the movement of heavenly bodies. It works well for big things. Likewise, public health is about populations, and these “big” concepts behave well at population levels. But populations are made of individuals, and there is still a gap between what we can predict at a population level and what we can predict at an individual level. When it comes to physics, Einstein figured out that things behave differently at the level of atoms of when things travel very fast. Like with physics, no one has been able to “unify” the theories of disease from population levels to personal levels.

For example, if we find that there is a statistically significant and causative association between a gene and a cancer, we might want to screen people for that gene. But finding that gene in a person doesn’t mean at all that the person is going to develop the cancer. (This is the argument behind HPV infection and cervical cancer. Having the virus doesn’t guarantee cancer. However, certain cancers almost always guarantee infection. It’s complicated, right?)

And this is where we are in public health. We need to always be balancing the needs of the many and the needs and realities of the few. We are okay with vaccinating millions even with the knowledge that a handful will have bad reactions to vaccines. (Though that number is still far smaller than some would have you believe.) We are also okay with dumping sewage out into oceans, rivers, and streams, because we haven’t seen effects at a large scale… Though we’ve seen effects at local and personal scales.

Whoever figures out a model for protecting, improving, and maintaining health at both the population level and the individual level will be remembered through history like we remember Snow, Pasteur, Koch, and Reed. That person will be a pioneer and visionary… And they’ll have to be incredibly smart. Putting all that together requires a special set of skills in solving puzzles.

I'm a doctoral candidate in the Doctor of Public Health program at the Johns Hopkins University Bloomberg School of Public Health. All opinions posted here are my own, of course, and they do not necessarily reflect the opinions of my school, employers, friends, family, etc. Feel free to follow me on Twitter: @EpiRen