**Core Concept**
Antigenic shift and drift are mechanisms by which the influenza virus evolves and changes its surface proteins, leading to new strains that can evade the immune system and cause infection in previously immunized individuals. Antigenic shift involves a major change in the virus's genetic material, resulting in a new subtype, whereas antigenic drift involves a minor change, leading to a new strain within the same subtype.

**Why the Correct Answer is Right**
Antigenic shift occurs when two different influenza viruses infect the same cell and undergo genetic recombination, resulting in a new virus with a mix of genetic material from both parent viruses. This process can happen in animals such as pigs, which can be infected by both human and avian influenza viruses. The new virus can then be transmitted to humans, leading to a pandemic. Antigenic drift, on the other hand, occurs through a process of mutation, where small changes occur in the virus's genetic material over time, resulting in a new strain.

**Why Each Wrong Option is Incorrect**
**Option A:** Influenza is a zoonotic disease, but antigenic shift and drift occur in the virus itself, not in the host.

**Option B:** Antigenic shift and drift are mechanisms of evolution specific to the influenza virus, not all viruses.

**Option C:** While antigenic shift and drift can lead to new strains of the virus, they are not the same as the process of viral replication.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that antigenic shift and drift are the primary reasons for the need for annual influenza vaccination and the development of new vaccines to match emerging strains.

**Correct Answer: C. Influenza virus.**