Pathogens are disease-causing microorganisms such as bacteria, viruses, fungi, and parasites. These microorganisms can infect humans, animals, and plants, causing a range of illnesses from mild to severe.

Examples of pathogens include:

- Bacteria: Salmonella, E. coli, Staphylococcus aureus
- Viruses: Influenza, HIV, Herpes simplex
- Fungi: Candida, Aspergillus
- Parasites: Malaria, Toxoplasma gondii, Giardia

Pathogens can be transmitted through various routes such as:

- Airborne: when an infected person coughs or sneezes
- Waterborne: when contaminated water is consumed
- Foodborne: when contaminated food is consumed
- Vector-borne: when an insect or other animal acts as a carrier of the disease

Once a pathogen enters the body, it can cause an infection by invading and multiplying within the host's cells or tissues. The body's immune system responds to the infection by producing antibodies to fight the pathogen. In some cases, the immune response can cause damage to the host's own tissues, leading to symptoms such as fever, inflammation, and pain.

In conclusion, pathogens are disease-causing microorganisms that can infect humans, animals, and plants through various routes of transmission. They can cause a range of illnesses from mild to severe and are a major public health concern worldwide.