Most people are born with normally functioning immune systems. The immune system functions to recognize an exposure to germs or disease. When our immune system sees this invasion of disease, our body will produce antibodies to fight off the disease. In most cases this process involves us getting sick, and showing symptoms caused by the particular disease. To avoid potentially deadly illness in this process, vaccines have been developed to provide immunity without needing to get sick in the first place.
In the United States, many diseases have been greatly reduced with preventative measures such as vaccinations. While traveling through other countries you can be exposed to diseases that are not commonly found in most areas of the United States. With less exposure to these diseases at home, most travelers will not have immunity or basic wellness training to prevent contracting these diseases.
Some preventable diseases that are commonly addressed for travel include: