A friend of mine said when she was a child she was very sickly and unwell. After contracting chicken pox she became much healthier and had a stronger constitution. She thought that this was probably because of the stimulation her immune system got from chicken pox. Contrarily my husband contracted chicken pox at 26 and was violently ill for about eight days. My child caught it twice at 18 months and again at 4 years and suffered very little from the infection.
Exposure to these illnesses allows our immune system to develop and build strength. Avoiding all illnesses (if this were ever possible) is not a good idea as it just leaves the immune system undeveloped.