A pathogen is a biological agent capable of producing disease in a host organism. Pathogens include viruses, bacteria, fungi, protozoa, helminths and prions.
Biological agents and disease mechanisms
Pathogens are categorized by their nature and structure. Viruses are acellular particles that hijack host cells to replicate. Bacteria are single-celled prokaryotes; some species cause disease by releasing toxins or by invading tissues. Fungi can cause infections of the skin, mucous membranes or systemic illnesses, particularly in immunocompromised hosts. Protozoa are unicellular eukaryotes that often have complex life cycles and may involve vectors; parasitic helminths are multicellular worms. Even misfolded proteins, known as prions, can induce neurodegenerative diseases in mammals. The ability of a pathogen to cause disease depends on factors such as virulence, host susceptibility and route of transmission. Many microbes are harmless or even beneficial under normal circumstances, but become pathogenic when they acquire virulence factors or when the host’s defenses are compromised. Understanding these mechanisms is central to infectious disease control, vaccine development and antimicrobial therapy.
Illustrative cases and notable diseases
Pathogens have shaped history through epidemics and chronic infections. Influenza viruses cause seasonal outbreaks and have been responsible for pandemics such as the 1918 flu and the 2009 H1N1 event. The SARS‑CoV‑2 virus emerged in 2019 and led to the COVID‑‑19 pandemic. Bacterial pathogens like Mycobacterium tuberculosis cause tuberculosis, while Vibrio cholerae produces cholera via toxin-mediated diarrhoea. Fungal pathogens such as Candida albicans lead to candidiasis, and Cryptococcus neoformans can cause meningitis in immunosuppressed individuals. Parasitic protozoa include Plasmodium species, which cause malaria, and Trypanosoma brucei, responsible for African sleeping sickness. Helminths such as Schistosoma mansoni contribute to schistosomiasis. Prions, although rare, cause fatal conditions like Creutzfeldt–Jakob disease. These examples highlight the diversity of pathogenic organisms and the need for surveillance, prevention and treatment strategies.
Pathogens influence human and animal health and ecosystems. By understanding their biology and interactions with hosts, scientists develop vaccines, diagnostics and control measures. Infection control practices and public health interventions reduce their impact.
Related Terms: Infectious agent, Virulence, Host, Disease, Microorganism