|My KIKO when he's around 2 year old|
Kids are prone to infections due to environmental factors, hygiene, activities such as playing and more often food intake.
Their immune systems are yet to mature. They are not exposed to as many bacteria and virus so they don't develop the antibodies that will fight those microorganisms. Although they have natural anti-bodies which are received through the placenta when they were born, they may acquire illness any time.
"When a baby is born, it has certain antibody-like proteins called immunoglobulins, which it has received through the placenta. Another very important one is received from its mother after it is born.
This immunoglobulin is called IgA and is present in the colostrum, the first part of mother's milk. It places itself on the mucus membranes of the baby's alimentary canal in order to help fight the bacteria that can cause gastroenteritis.
Babies do not start to produce their own IgA until around 6 months of age. By their first birthday they have between 15-17% of their adult level." - SOURCE