**Core Concept**
Hydatid disease, also known as echinococcosis, is a parasitic infection caused by the Echinococcus tapeworm. The life cycle of this parasite involves two hosts: an intermediate host and a definitive host. The intermediate host is crucial for the development of the parasite's larval stage, which can cause cysts in various organs.

**Why the Correct Answer is Right**
The intermediate host for hydatid disease is typically a herbivore, such as sheep, goats, or cattle. When these animals ingest eggs of the Echinococcus tapeworm, the eggs hatch into larvae that migrate to various organs, including the liver, lungs, or other internal organs, and form cysts. This process is facilitated by the parasite's ability to evade the host's immune system and use enzymes to break down tissues and create a suitable environment for growth.

**Why Each Wrong Option is Incorrect**
**Option A:** Humans are not considered intermediate hosts, although they can act as accidental hosts if they ingest contaminated food or water.
**Option B:** Dogs and other canines are definitive hosts, not intermediate hosts, as they harbor the adult tapeworms in their intestines.
**Option C:** Pigs are not typically considered intermediate hosts for hydatid disease, although they can be infected under certain circumstances.

**Clinical Pearl / High-Yield Fact**
The intermediate host is essential for the development of the parasite's larval stage, which can cause cysts in various organs. This highlights the importance of understanding the life cycle of the Echinococcus tapeworm in controlling and preventing hydatid disease.

**Correct Answer: D. Sheep.**