Pathogens and how diseases spread
Infection and response • Communicable diseases
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Key concepts
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Definition of a pathogen
A pathogen is any microorganism that transmits a communicable disease from one organism to another. Pathogens include viruses, bacteria, fungi and protists and share a simple life cycle: infect a host, reproduce or replicate, then spread to new hosts . Many pathogens survive briefly outside a host but require a host cell to reproduce; viruses replicate inside host cells, while many bacteria and protists reproduce independently when conditions allow .
Types of pathogen and limiting factors
Viruses are small particles of genetic material inside a protein coat; they replicate only inside host cells and do not carry out full life processes, which limits their survival outside hosts . Bacteria are single-celled organisms that reproduce by binary fission and can survive in varied environments; some form resistant strains that survive longer outside hosts . Fungi include single-celled and multicellular forms that produce spores and grow as hyphae; many fungal infections need warm, moist conditions to spread, limiting transmission in dry environments . Protists are eukaryotic microorganisms that often require vectors or water for transmission; many protist pathogens cause diseases such as malaria and need specific environmental conditions for the vector to survive .
Main transmission routes
Direct contact transmits pathogens when infected tissue or bodily fluids touch a new host; sexually transmitted bacterial infections and skin-to-skin spread of fungal infections are examples . Airborne transmission occurs when tiny droplets containing pathogens form during coughing or sneezing and are inhaled by others; measles spreads effectively by this route because droplets remain infectious in the air . Waterborne transmission occurs when pathogens contaminate drinking or bathing water; cholera and many diarrhoeal diseases spread through unsterilised or sewage-contaminated water, so clean water and sewage treatment reduce transmission . Vectors such as mosquitos carry some protists and viruses from host to host; malaria transmission requires the mosquito vector, so vector presence limits disease distribution .
Spread in animals
Animal infections spread by contact (touch, sexual contact), airborne droplets, contaminated food or water, infected vectors, and contaminated surfaces (fomites). Each route causes infection when pathogens reach susceptible tissues and encounter favourable conditions to reproduce or replicate . Environmental and social factors limit spread: low temperature can prevent vector survival (reducing malaria risk), good sanitation reduces waterborne disease, and immunity in the population reduces transmission of airborne viruses such as measles .
Spread in plants
Plant pathogens include viruses, bacteria and fungi. Transmission occurs through water and air, by insect vectors or worms, via seeds and pollen, and by direct contact between infected and healthy plants . Soil-borne pathogens can survive outside a host if environmental temperature and conditions permit, allowing infection of new plants planted in the same soil . Plant disease control often targets the transmission route: removal and burning of infected leaves prevents fungal spread, and chemical sprays target fungal or bacterial pathogens to limit reproduction and further infection .
Prevention and control measures
Vaccination induces immunity in hosts and reduces the number of susceptible individuals, lowering transmission of many viral diseases; measles vaccination reduces airborne spread by producing immune individuals who cannot transmit infection . Hygiene measures such as handwashing, use of antiseptics and cleaning surfaces reduce transmission by direct contact or contaminated surfaces; antiseptics and disinfectants kill pathogens on skin and surfaces and reduce onward spread . Water treatment and sanitation prevent waterborne disease by removing pathogens from drinking water and sewage; sewage treatment centres reduce contamination of water supplies and thereby cut transmission of organisms such as cholera . Vector control (insect nets, insecticides, reducing standing water) reduces diseases that require vectors, and targeted plant measures (destroy infected material, fungicide sprays) limit agricultural losses from plant pathogens .
Key notes
Important points to keep in mind