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Pathogens are disease-causing microorganisms that can infect living organisms. These microscopic agents include bacteria, viruses, fungi, and protozoa. They are a natural part of the environment but can pose significant health risks when they enter a host's body and disrupt its normal functions.

Bacteria are single-celled organisms that can reproduce independently. Some bacteria are beneficial, aiding in digestion or producing antibiotics, while others, like Streptococcus pneumoniae, can cause infections such as pneumonia. Viruses are even smaller than bacteria and require a host cell to replicate. They can cause a wide range of illnesses, from the common cold (rhinoviruses) to more severe diseases like HIV/AIDS (human immunodeficiency virus).

Fungi are eukaryotic organisms that can exist as yeasts or molds. While many fungi are harmless or even beneficial, some, like Candida albicans, can cause infections, particularly in individuals with weakened immune systems. Protozoa are single-celled eukaryotes, some of which are parasitic and transmitted through contaminated water or insects. Examples include Plasmodium species, which cause malaria.

The transmission of pathogens can occur through various routes, including direct contact, airborne droplets, contaminated food and water, and vectors like insects. Understanding how pathogens spread is crucial for developing effective prevention and treatment strategies, such as hygiene practices, vaccination, and antimicrobial therapies.