What causes disease? That might sound like a basic question, but the answer is far from simple. In the past, many diseases were blamed on a single factor, such as a virus, bacteria, or nutritional deficiency. These causes were clear and direct. Think of scurvy caused by lack of vitamin C or tuberculosis caused by a specific bacterium.
But things have changed. Today, more people die from chronic noncommunicable diseases than from infections, especially in wealthier countries. These include heart disease, diabetes, some cancers, and mental health disorders. And when we ask, “What causes these?” the answer becomes messy.
Chronic Diseases and Their Complicated Roots
More Than One Cause
Chronic noncommunicable diseases (CNCDs) don’t come from one clear source. Instead, they usually arise from a combination of factors working together. Take heart disease as an example. A poor diet, smoking, stress, lack of exercise, and family history can all play a role. No single factor alone explains why someone gets sick.
This makes the job of epidemiologists more difficult. They used to study clear outbreaks like cholera or polio. Now, they’re trying to untangle webs of overlapping causes that play out slowly over years. The way we live, the air we breathe, the jobs we do, and even the social systems we live under all become part of the equation.
The “Nature and Nurture”
To help make sense of this, many epidemiologists use a simple idea, nature loads the gun, nurture pulls the trigger. In other words, our genes may make us more vulnerable to certain diseases, but it’s often environmental or lifestyle factors that set the disease in motion.
For example, someone may have genes that increase their risk of developing Type 2 diabetes. But without certain lifestyle conditions, like poor diet, high stress, or sedentary behavior, that person might never actually develop the disease. The genetic risk sits quietly in the background until the right (or wrong) environmental factors appear.
When Genetics Enters the Picture
A New Challenge for Epidemiology
Epidemiology used to focus mostly on external factors. Things like food, pollution, and pathogens. But now, it also has to handle internal factors like genes. This changes everything. Genes are not easily visible, and their effects often depend on how they interact with the environment.
One example is breast cancer. Some women carry mutations in a gene called BRCA1. This greatly increases their risk, but not every woman with the mutation gets cancer. Whether the disease shows up may depend on environmental exposures, hormone levels, or even social stress.
The interaction between genes and environment creates a complex puzzle. It’s no longer enough to study only one side of the picture. Modern epidemiologists need to combine biological knowledge with social, environmental, and even psychological insights.
A Moving Target
Another challenge is that the causes of disease can change over time. Smoking is a clear example. In the early 1900s, hardly anyone suspected it was harmful. Today, it’s one of the most studied risk factors in public health history. What we understand as “causes” is always growing and shifting.
We also now recognize causes that are harder to see, like long-term exposure to air pollution or the effects of childhood trauma on adult health. These kinds of discoveries make epidemiology both exciting and demanding. As new causes are uncovered, the framework of how we understand disease must adjust.
Building a Better Framework for Modern Health
Moving Beyond the One-Cause Model
To respond to these challenges, epidemiology is slowly moving away from the idea that a single cause leads to a single disease. Instead, researchers are building models that accept multiple contributing factors. These models might show how poverty increases stress, which affects hormones, which in turn may raise the risk of heart disease. It’s a layered view of causation that includes biology, environment, and society.
This new approach also pushes public health toward more holistic interventions. For instance, solving a community’s high diabetes rate might mean more than just educating people about sugar. It could also mean improving access to healthy food, reducing neighborhood violence, and creating safer spaces for exercise.
How is this important to epidemiologists?
Epidemiology today is not about blaming individuals. It’s about uncovering the full picture so that people, communities, and governments can make better decisions. When we understand that both our biology and our environment matter, we can build health systems that treat people more fairly and more effectively.