External factors responsible to produce disease is

Options

A. virus-infected cells
B. bacteira
C. parasites
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding External Factors**: External factors are agents or elements from the environment that can cause diseases in humans. These factors can include microorganisms and other environmental elements.

2. **Identifying Types of External Factors**:
- **Viruses**: These are microscopic pathogens that invade human cells, leading to infections. When a virus infects a healthy cell, it can turn that cell into a virus-infected cell, which can then spread the infection to other healthy cells.
- **Bacteria**: These are single-celled organisms that can enter the body and multiply. Some bacteria are harmful and can cause diseases by destroying human cells or producing toxins.
- **Parasites**: These are organisms that live on or in a host and benefit at the host's expense. They can absorb nutrients from the host, leading to various health issues.

3. **Additional External Factors**:
- **Fungi**: Certain fungi can also cause diseases, particularly in individuals with weakened immune systems.
- **Environmental Elements**: Factors such as contaminated water, soil, and air can harbor pathogens like bacteria and viruses, which can lead to diseases when they enter the human body.

4. **Conclusion**: The external factors responsible for producing diseases include viruses, bacteria, parasites, fungi, and environmental contaminants. All of these can lead to various health issues in humans.