The third step is to design and test out interventions or treatments to see what works best. If you were developing a vaccine or drug treatment this might involve a small scale trial to start with.
Vaccinations are probably one of the best known public health measures.

Since we are in the middle of the COVID pandemic it’s useful to think about how vaccines are used to prevent disease. Vaccination aims to prevent infectious diseases caused by viruses
Vaccinations not only prevent or reduce the severity of the disease in the person receiving the vaccination, they also help to protect other people in society who might not be able to be vaccinated by reducing the spread of disease.
They can also help to eradicate a disease all together if enough people are vaccinated -this is called herd immunity. Over the years vaccination has completely eradicated diseases such as small pox and made the occurrence of diseases like polio and tetanus very rare. Other diseases like measles are seen much less often than they used to be.
Vaccines are thought to prevent 3 million deaths a year worldwide.
The way vaccines work to make someone immune to a disease that is preventable is often applied to violence prevention.