Normally, when a weakened version of a virus is introduced into the body, the immune system not only reacts to destroy it but also constructs defenses that will successfully defeat any invasion by the real virus.
पीत - ज्वर, खसरे, गलसुए, हल्के खसरे के असरदार टीके बनाने के लिए इन्हीं बीमारियों के कमज़ोर वायरसों का इस्तेमाल किया जाता है ।