Live vaccines are –
**Core Concept**
Live vaccines contain a weakened or attenuated form of the disease-causing microorganism, which allows the body to mount an immune response without causing the full-blown disease. This type of vaccine is particularly effective in providing long-term immunity and is often used to prevent serious infections such as measles and mumps.
**Why the Correct Answer is Right**
The attenuation process involves reducing the virulence of the microorganism through various methods, such as serial passage in a non-human host or treatment with chemicals that selectively kill virulent cells. This weakened form of the microorganism is then used to infect the vaccine recipient, who mounts an immune response against it. The immune system produces antibodies and immune cells that can recognize and attack the weakened microorganism, providing immunity against future infections.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because inactivated vaccines, not live vaccines, are made from killed or inactivated microorganisms. Inactivated vaccines are often used for individuals with weakened immune systems.
* **Option B:** This option is incorrect because toxoids are vaccines that contain inactivated toxins produced by microorganisms, rather than the microorganisms themselves. Toxoids are used to provide immunity against diseases caused by bacterial toxins, such as tetanus and diphtheria.
* **Option C:** This option is incorrect because subunit vaccines contain only a portion of the microorganism, such as a protein or sugar, rather than the entire microorganism. Subunit vaccines are often used to provide immunity against diseases such as HPV and Hib.
**Clinical Pearl / High-Yield Fact**
Live vaccines can cause mild side effects, such as fever and rash, due to the replication of the attenuated microorganism in the body. However, these side effects are typically mild and short-lived, and are often a sign that the vaccine is working.
**Correct Answer:** C.