## **Core Concept**
Newcastle disease conjunctivitis, also known as Newcastle disease or Avian pneumovirus infection, is an infectious disease primarily affecting birds but can cause conjunctivitis in humans through contact with infected birds or contaminated materials. The causative agent is a paramyxovirus.

## **Why the Correct Answer is Right**
The correct answer, **D. Birds**, is right because Newcastle disease is a viral infection that primarily affects birds, causing severe illness in poultry. Humans can contract the disease, manifesting as conjunctivitis, through direct contact with infected birds or their secretions. The virus can be transmitted through respiratory droplets, contaminated feed and water, or direct contact with infected tissues.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Incorrect because the disease is not primarily spread through contact with diseased humans; it's an avian disease with occasional zoonotic transmission.
- **Option B:** Incorrect as there is no common association of Newcastle disease with infected fish that would lead to conjunctivitis in humans.
- **Option C:** Incorrect because, although the disease can potentially be spread through fomites (inanimate objects) contaminated with the virus, the primary source of infection for humans is direct contact with diseased birds.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that individuals working with birds, such as farmers, poultry workers, or veterinarians, are at risk of developing Newcastle disease conjunctivitis. A classic presentation is unilateral conjunctivitis following exposure to infected birds. The disease is usually self-limiting in humans.

## **Correct Answer:** **D. Birds**