To help explain the difference, take this example. Let’s say you are the mayor of a town near a swimming hole used by kids and adults alike. One summer, you learn that citizens are developing serious and persistent rashes after swimming as a result of a chemical irritant in the river. You decide to take action.
If you approach the company upstream that is discharging the chemical into the river and make it stop, you are engaging in primary prevention. You are removing the hazardous exposure and preventing rashes in the first place.
If you ask lifeguards to check swimmers as they get out of the river to look for signs of a rash that can then be treated right away, you are engaging in secondary prevention. You are not preventing rashes, but you are reducing their impact by treating them early on so swimmers can regain their health and go about their everyday lives as soon as possible.
If you set up programs and support groups that teach people how to live with their persistent rashes, you are engaging in tertiary prevention. You are not preventing rashes or dealing with them right away, but you are softening their impact by helping people live with their rashes as best as possible.
For many health problems, a combination of primary, secondary and tertiary interventions are needed to achieve a meaningful degree of prevention and protection. However, as this example shows, prevention experts say that the further “upstream” one is from a negative health outcome, the likelier it is that any intervention will be effective.