**Core Concept**
Antiserum is a type of passive immunization used to provide immediate protection against specific diseases by transferring pre-formed antibodies from an immunized individual or animal. This concept is based on the principle of antibody-mediated immunity, where pre-formed antibodies recognize and neutralize pathogens, thereby preventing infection.

**Why the Correct Answer is Right**
Passive immunization with antiserum is particularly useful in situations where active immunization is not feasible, such as in emergency situations or in individuals with compromised immune systems. Antiserum is prepared by collecting blood from animals immunized with a specific antigen, and the antibodies present in the blood are then purified and administered to the individual in need. This approach is effective against diseases caused by bacterial toxins, such as diphtheria and tetanus, as well as against certain viral infections.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because antiserum is not typically used for immunization against viral infections such as HIV, where active immunization is the preferred approach.
**Option B:** This option is incorrect because antiserum is not commonly used for immunization against bacterial infections such as pneumonia, where antibiotics are the primary treatment.
**Option C:** This option is incorrect because antiserum is not used for immunization against fungal infections, where antifungal medications are the primary treatment.

**Clinical Pearl / High-Yield Fact**
When considering passive immunization with antiserum, it is essential to remember that the duration of protection is limited to the lifespan of the administered antibodies, typically ranging from a few days to several weeks. This highlights the importance of repeated administration or active immunization to maintain long-term protection.

**Correct Answer: D. Tetanus. Tetanus antiserum is used for passive immunization against tetanus toxin.**