What is a pathogen?

Explanation

**Step-by-Step Solution:**

1. **Definition of Pathogen**: A pathogen is defined as a living organism that causes disease. This can include various types of microorganisms that can infect living hosts, including humans, animals, and plants.

2. **Types of Pathogens**: Pathogens can be classified into several categories based on their nature. The main types include:
- **Viruses**: These are tiny infectious agents that can only replicate inside the living cells of an organism. An example of a viral pathogen is HIV, which causes AIDS.
- **Bacteria**: These are single-celled organisms that can cause diseases. For instance, Mycobacterium causes tuberculosis, and Salmonella typhi causes typhoid fever.
- **Protozoa**: These are single-celled organisms that can also lead to diseases. An example is Plasmodium, which causes malaria.
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