The term 'immunity' refers to

Answer

**Step-by-Step Text Solution:**

1. **Understanding the Term 'Immunity':**
- The term 'immunity' refers to the body's ability to resist and fight against disease-causing organisms, also known as pathogens.

2. **Analyzing the Options:**
- **Option A:** Mutualism between host and parasite.
- This describes a relationship where both organisms benefit, which does not relate to immunity. In fact, mutualism is not applicable here as immunity is about defense against pathogens.
- **Option B:** Ability of the host to fight the disease-causing organisms.
- This accurately describes immunity. The immune system is responsible for identifying and combating pathogens, making this option correct.
- **Option C:** Ability of the parasites.
- This refers to the capacity of parasites to survive and reproduce within a host, which is not related to immunity.

3. **Conclusion:**
- The correct answer is **Option B**, as it directly defines immunity as the ability of the host to fight against disease-causing organisms.