There are various ways of transmission of diseases. Some organisms transmit diseases from person to person. Carriers of specific germs are called vectors. Anopheles is a vector for malarial parasite, Plasmodium . Similarly, Aedes aegypti is a vector for dengue virus.

A vaccine is a suspension of disease-producing microorganisms which is modified by killing or weakening so that the suspension will not cause disease. Rather it stimulates the formation of antibodies upon inoculation. The antibodies remain in blood for long and when the germs of a particular disease enter the body, the antibodies destroy them. This is the basis of immunisation.

Vaccination is a simple, safe, and effective way of protecting people against harmful diseases, before they come into contact with them. Vaccines work by training and preparing the body's natural defences --- the immune system--- to recognize and fight off the pathogens like viruses and bacteria they target. It is best for a child to have the vaccine within a few days of being born and up to six months old, but they can be vaccinated any time up to five years of age.