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Pathogens are microorganisms or infectious agents that can cause disease in their host organisms. These agents include bacteria, viruses, fungi, protozoa, and prions, each capable of invading living tissues, multiplying, and disrupting normal physiological functions. Pathogens spread through various routes, such as direct contact, airborne transmission, contaminated food or water, and vectors like insects or animals.

Bacteria are single-celled organisms that reproduce rapidly and can release toxins harmful to the host. Examples include *Streptococcus* (responsible for strep throat) and *Escherichia coli* (linked to foodborne illnesses). Viruses are smaller than bacteria and require a host cell to replicate; common viruses include influenza, HIV, and SARS-CoV-2, which causes COVID-19. Fungi, such as *Candida albicans*, can cause infections like thrush or athlete’s foot, particularly in immunocompromised individuals. Protozoa, like *Plasmodium*, cause malaria by infecting red blood cells, while prions are misfolded proteins that lead to neurodegenerative diseases such as Creutzfeldt-Jakob disease.

Pathogens exhibit varying degrees of virulence, or disease-causing potential, and can be classified as opportunistic or obligate pathogens. Opportunistic pathogens, such as certain fungi or bacteria, exploit weakened immune systems to infect the host, whereas obligate pathogens, like tuberculosis bacteria, require a host to survive and replicate. Factors influencing pathogenicity include the host’s immune response, environmental conditions, and the pathogen’s ability to evade defenses.

Prevention and control of pathogen transmission rely on hygiene practices, vaccination, antibiotics (for bacterial infections), and public health measures like sanitation and quarantine. Understanding pathogen biology and epidemiology is essential for developing effective treatments and mitigating outbreaks. Research in microbiology and immunology continues to advance our ability to combat infectious diseases and improve global health outcomes.