Epidemiologists study how diseases and health conditions spread through populations, what causes them, and how to prevent them. Their work spans everything from tracking a foodborne illness outbreak at a single restaurant to analyzing decades of data on heart disease risk across entire countries. In the United States alone, about 12,300 people work as epidemiologists, most of them employed by state and local governments.
The Core Work of Epidemiology
Epidemiology has six core tasks: public health surveillance, field investigation, analytic studies, evaluation of health programs, building linkages across disciplines, and policy development. Surveillance is the backbone. It means continuously collecting and analyzing health data so that when something unusual appears, like a spike in emergency room visits or a cluster of a rare cancer, someone notices early enough to act.
Field investigation is what happens next. A single case report might trigger nothing more than a phone call to a doctor’s office to verify details. A cluster of unexplained pneumonia cases, on the other hand, could mobilize dozens of people to determine the scope and source of the problem. The CDC teaches a formal 10-step process for these investigations, starting with confirming the diagnosis and counting cases, then organizing findings by time, place, and person, developing hypotheses, running systematic studies, and implementing control measures.
Epidemiologists also design and run analytic studies when surveillance and fieldwork alone can’t pin down a cause. And because they’re trained to think in systematic, quantitative terms, they frequently evaluate whether public health programs are actually working. Field epidemiology is often called a “team sport” because the work almost always involves collaboration with lab scientists, clinicians, policymakers, and other specialists.
Infectious Disease Tracking
Infectious disease is the most visible branch of epidemiology. When a new pathogen emerges or an old one resurfaces, epidemiologists map transmission patterns: who got sick, where, and from what source. They distinguish between infection and disease, a distinction that matters more than most people realize. Someone with latent tuberculosis, for example, tests positive on a skin test but has no symptoms and isn’t contagious. Someone with active TB coughs, loses weight, and can spread the bacteria to others. Epidemiologists track both groups because latent cases can become active ones.
Whole-genome sequencing has transformed this work in recent years. By reading the genetic code of a pathogen, epidemiologists can now trace transmission chains with far more precision than older methods allowed. They can determine whether two patients in different hospitals caught the same strain, identify how a virus is evolving over time, and even detect rising variant prevalence through wastewater sampling before clinical testing picks it up. During the COVID-19 pandemic, wastewater sequencing flagged the Omicron lineage in several regions earlier than hospital data did, giving public health teams a head start on targeted testing. National genomic surveillance programs now inform vaccine strain selection for influenza and SARS-CoV-2 alike.
Chronic Disease and Lifestyle Risk Factors
Not all epidemiology involves outbreaks. Chronic diseases like heart disease, cancer, and diabetes are the leading causes of death and disability in the United States, and epidemiologists have spent decades identifying the behaviors that drive them. The list is surprisingly short: smoking, poor nutrition, physical inactivity, and excessive alcohol use account for most preventable chronic disease.
Smoking alone causes cancer, heart disease, stroke, lung disease, diabetes, and chronic obstructive pulmonary disease. Poor diet and lack of exercise are major risk factors for obesity, type 2 diabetes, heart disease, stroke, certain cancers, and depression. Excessive alcohol use is linked to high blood pressure, liver disease, stroke, and several types of cancer. Epidemiologists established these connections through long-running population studies that followed thousands of people over years or decades, tracking who got sick and what habits they had in common.
Environmental and Social Factors
Environmental epidemiologists study how exposures in the world around you affect health. Established links include radon and lung cancer, arsenic and cancers in multiple organs, lead and nervous system disorders, airborne particulate matter and worsening cardiovascular and respiratory disease, and waterborne bacteria like E. coli causing gastrointestinal illness. These relationships take years to document because exposures are often low-level and chronic, making them hard to isolate from other variables.
Social epidemiology takes a wider lens. Researchers in this field examine how income, education, and employment shape health outcomes. The pattern is remarkably consistent: people with fewer socioeconomic resources get sicker and die younger, across virtually every health measure studied. Health-related behaviors like diet, exercise, and smoking are themselves strongly shaped by social factors. Some critics have argued that the link runs in reverse, that sickness leads to lost income rather than the other way around. But the weight of evidence supports both directions, with social disadvantage acting as what researchers call “a cause of the causes.”
How Epidemiologists Measure Disease
Two numbers sit at the center of epidemiological thinking: incidence and prevalence. Incidence is the rate of new cases appearing in a population over a specific time period. If 500 people in a city of 100,000 are diagnosed with the flu during January, that’s an incidence rate. Only people who didn’t already have the flu count in the population at risk, because you can’t newly develop something you already have.
Prevalence is different. It captures how many people have a condition at a given moment, regardless of when it started. A disease with low incidence but long duration, like diabetes, will have high prevalence because cases accumulate over time. A disease that strikes fast and resolves quickly, like norovirus, will have low prevalence even if incidence is high. Epidemiologists choose between these measures depending on the question. Incidence helps you understand risk. Prevalence helps you understand burden.
Study Designs Epidemiologists Use
The type of study an epidemiologist runs depends on the question, the timeline, and the resources available. In a cohort study, researchers follow a group of people over time to see who develops a condition and what exposures they had. These are powerful for establishing cause and effect, but they’re expensive and slow. In a case-control study, researchers start with people who already have the disease and look backward to compare their exposures to those of healthy people. This approach is faster and cheaper, and especially useful for studying rare diseases.
Cross-sectional studies take a snapshot. Everyone is measured at a single point in time, which is efficient for estimating how common a condition is but weak for proving what caused it. You can see that two things occur together, but you can’t tell which came first. Each design has trade-offs, and epidemiologists often use multiple approaches to build a more complete picture of the same health problem.
Where Epidemiologists Work
The majority of epidemiologists work in government. About 34% are employed by state government agencies and another 22% by local government, typically in health departments conducting surveillance and responding to outbreaks. Hospitals employ roughly 10%, scientific research organizations about 5%, and colleges and universities another 5%. Salaries vary by sector: epidemiologists in scientific research earn the most, averaging about $127,000 per year, while those in state and local government average around $79,000. The work looks different depending on the setting. A state health department epidemiologist might spend the week analyzing reportable disease data and coordinating a response to a salmonella cluster. An academic epidemiologist might spend years running a cohort study on air pollution and childhood asthma.