**Core Concept**
Live vaccines are used for certain infectious diseases to stimulate a mild, controlled infection that induces long-lasting immunity. They contain a weakened or attenuated form of the pathogen, which replicates in the body without causing severe illness.

**Why the Correct Answer is Right**
Live vaccines are typically used for diseases where a strong immune response is desired, and the risk of severe illness from the wild-type pathogen is low. Examples of live vaccines include the MMR (measles, mumps, and rubella) vaccine, the varicella (chickenpox) vaccine, and the oral poliovirus vaccine (OPV). These vaccines replicate in the body and provide long-lasting immunity against the targeted pathogens.

**Why Each Wrong Option is Incorrect**
**Option A:** Killed vaccines, such as those for hepatitis A and typhoid, are used for diseases where a strong immune response is not required or where the risk of severe illness from the wild-type pathogen is high.
**Option B:** Toxoids, like tetanus and diphtheria toxoids, are used to protect against diseases caused by toxins produced by bacteria, and they do not contain live pathogens.
**Option C:** Conjugate vaccines, such as those for pneumococcal disease and meningococcal disease, use a combination of a polysaccharide antigen and a protein carrier to stimulate an immune response, but they are not live vaccines.

**Clinical Pearl / High-Yield Fact**
Live vaccines should be used with caution in immunocompromised individuals, as they can cause severe illness in people with weakened immune systems.

**Correct Answer:** C. Oral poliovirus vaccine (OPV).