Vaccination is one of the best ways to protect the body against disease. When someone is vaccinated, elements of a disease that has been weakened or inactivated are injected into the body. This stimulates the immune system to fight the invading disease. After the immune system has overcome the disease, it creates cells that stay alert for the disease-causing microbes, making the body immune to that particular disease.

In some cases, immunity lasts for life. However, some vaccines require a booster shot some time later.

Different types of vaccines are used to protect against different diseases. Types of vaccine include:

• Live vaccine. Contains live versions of a virus or bacterium that have been weakened to not cause disease in the body. This produces a strong immune reaction and in many cases protects for life.
  

  

• Inactivated vaccine. Contains a dead version of the virus or bacterium. Inactivated vaccines may not produce a strong immune reaction and booster shots are sometimes required.
  
  
• Subunit vaccine. Contains only the antigens of the disease-causing microbe, which reduces the risk of the body reacting adversely to the vaccine.
  
  
• Toxoid vaccine. Contains the toxins that are sometimes secreted by bacteria. A toxoid vaccine only protects against diseases caused by certain toxins.
  
  
• Conjugate vaccine. Designed for infants and young children. A conjugate vaccine protects infants and young children from microbes that may take advantage of their immature immune systems.

In addition to these types of vaccinations, scientists are experimenting with vaccines that contain the DNA of diseases. These vaccines may be able to protect people against diseases for which there are currently no vaccines available, such as HIV and malaria.

Most vaccinations are given during early childhood. Vaccines are most effective when they are given at the earliest point that the immune system can respond effectively to a disease-causing microbe. Additionally, children are also more vulnerable to many diseases than adults. Childhood vaccination is one of the ways that diseases such as measles, which used to be one of the most common and deadly diseases among children in the United States, have been essentially eradicated.

People who travel to other countries may need vaccinations to protect them against diseases that are uncommon in the United States. Diseases that travelers are commonly vaccinated against include yellow fever, typhoid fever and hepatitis A.