There are many reasons people become sick here are a few.
1) Infection by viruses eg chikenpox, 'flu, the common cold.
2) Infection by bacteria eg leprosy, staphlococcal boils, tetanus.
3) Karyotype differences eg Down's syndroms, Kleinfelter's syndrome,
4) Genetic mutations eg sickle cell anemia,
5) Genetic predispositions
6) Physical wear and tear eg broken bones, cuts and abrasions.
8)Chemical assault on the body eg cancers caused by some chemicals, chemical burns.
9) UV assault on the body eg skin cancers