Antibiotics
Antibiotics were invented to fight against serious infections and diseases. They help the antibodies in the body. However, when these are abused or taken too often the body can build up a resistance against them, people who take antibiotics often have less immunity and are sick more than people who take them on the odd occasion. It is important to always ensure they are necessary and prescribed.
Age
Age has a profound effect on the immune system. Babies do not have fully developed antibodies just yet, thus they need vaccinations and simply time to develop their immune systems fully. The elderly on the other hand have decreased immunity as their antibodies have become weaker with time and age.