Pathogen

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Infectious agents that can cause disease in humans, animals, plants, and other hosts

The host-pathogen interface includes interactions among an infectious agent, host defenses, and environmental conditions.

Pathogens are infectious agents that can cause disease in a host organism. They include viruses, bacteria, fungi, parasites, and prions, as well as some infectious agents that are difficult to classify. Pathogens can infect humans, animals, plants, and microorganisms, and the diseases they cause range from mild self-limited infections to severe, life-threatening illnesses such as sepsis, meningitis, pneumonia, malaria, tuberculosis, COVID-19, and Creutzfeldt-Jakob disease."The Pathogenic Potential of a Microbe".mSphere.2017;2(1)doi:10.1128/mSphere.00015-17.PMC:5326610.Infectious diseases - Symptoms and causes(link). Mayo Clinic.

A pathogen's ability to cause disease depends not only on the organism itself, but also on the host's immune system, the route of exposure, the infectious dose, the presence of virulence factors, environmental conditions, and access to prevention or treatment. This relationship is often described through the epidemiological triad of agent, host, and environment."Principles of Infectious Diseases: Transmission, Diagnosis, Prevention, and Control".International Encyclopedia of Public Health.2017;

22-39.doi:10.1016/B978-0-12-803678-5.00516-6.PMC:7150340.

Overview[edit]

Pathogens are central to the study of infectious disease, microbiology, immunology, epidemiology, public health, infection prevention, and antimicrobial therapy. Some microbes are always associated with disease when found in certain body sites, while others are normally harmless or beneficial but can become pathogenic under specific conditions. These are called opportunistic pathogens.

A microorganism may cause disease by:

  • Invading host tissues.
  • Damaging cells directly.
  • Producing toxins.
  • Triggering excessive inflammation.
  • Evading the immune system.
  • Persisting inside cells.
  • Disrupting normal microbial communities.
  • Causing chronic immune activation.
  • Spreading through blood or lymph to distant organs.

Not all exposure to a pathogen results in illness. Some exposures lead to asymptomatic infection, colonization, latent infection, carrier states, or immunity without disease.

Terminology[edit]

  • Pathogen - Infectious agent capable of causing disease.
  • Microorganism - Microscopic organism, including bacteria, fungi, protozoa, and some algae.
  • Microbe - General term for microscopic life forms and infectious agents.
  • Infectious agent - Organism or particle capable of causing infection.
  • Infection - Entry and multiplication of a pathogen in or on a host.
  • Colonization - Presence of microbes without tissue invasion or symptoms.
  • Disease - Harmful host response or tissue damage producing signs or symptoms.
  • Pathogenicity - Ability of an organism to cause disease.
  • Virulence - Degree of pathogenicity or severity caused by a pathogen.
  • Virulence factor - Molecule or trait that helps a pathogen cause disease.
  • Opportunistic pathogen - Organism that causes disease mainly when host defenses are impaired.
  • Commensal organism - Microbe living on or in the host without causing harm.
  • Microbiome - Community of microorganisms in a body site or environment.
  • Carrier state - Host harbors and may transmit pathogen without symptoms.
  • Reservoir - Natural habitat or host where a pathogen persists.
  • Vector - Organism such as a mosquito or tick that transmits a pathogen.
  • Zoonosis - Infection transmitted between animals and humans.
  • Emerging infectious disease - New or increasing infectious disease threat.
  • Antimicrobial resistance - Ability of pathogens to survive drugs designed to kill or inhibit them.

Major types of pathogens[edit]

Viruses[edit]

Viruses are small infectious agents that require living cells to replicate. They contain genetic material, either DNA or RNA, enclosed in a protein coat and sometimes a lipid envelope. Viruses are obligate intracellular parasites, meaning they must use host-cell machinery to reproduce.

Common viral diseases include:

Important viral concepts include:

Bacteria[edit]

Bacteria are single-celled prokaryotes. Many bacteria are harmless or beneficial, but some cause disease. Bacterial pathogens may invade tissues, produce toxins, form biofilms, or trigger destructive inflammation.

Common bacterial diseases include:

Important bacterial concepts include:

Fungi[edit]

Fungi are eukaryotic organisms that include yeasts, molds, and dimorphic fungi. Fungal diseases range from superficial skin infections to invasive infections in immunocompromised patients. Fungal diseases are increasing worldwide, and some fungi are becoming resistant to antifungal medications.Fungal Diseases(link). Centers for Disease Control and Prevention.

Common fungal diseases include:

Important fungal concepts include:

Parasites[edit]

Parasites are organisms that live on or in a host and obtain nutrients at the host's expense. CDC defines parasites as organisms that live on or in a host organism and get food from or at the expense of that host.Parasites(link). Centers for Disease Control and Prevention.

Major parasite groups include:

Common parasitic diseases include:

Important parasitology concepts include:

Prions[edit]

Prions are infectious misfolded proteins that cause fatal neurodegenerative diseases. Unlike bacteria, viruses, fungi, and parasites, prions do not contain DNA or RNA. CDC notes that prion diseases occur when normally occurring proteins misfold and cause brain damage, and that once symptoms develop, disease progresses rapidly and is fatal.About Prion Diseases(link). Centers for Disease Control and Prevention.

Prion diseases include:

Important prion concepts include:

Other infectious agents[edit]

Some infectious entities do not fit neatly into the major groups.

  • Viroid - Small infectious RNA molecule affecting plants.
  • Satellite virus - Virus-like agent requiring helper virus.
  • Endosymbiont - Organism living inside another organism, sometimes pathogenic depending on context.
  • Algae - Rarely pathogenic in humans, but some species can cause disease or toxin exposure.
  • Toxin-producing microorganism - Microbe that causes illness primarily through preformed toxins.

Pathogenicity and virulence[edit]

Pathogenicity is the ability of a pathogen to cause disease. Virulence describes the severity or degree of disease-causing ability. A highly virulent pathogen may cause severe disease, rapid tissue damage, high mortality, or efficient spread.

Factors influencing pathogenicity and virulence include:

  • Infectious dose.
  • Route of entry.
  • Ability to attach to host cells.
  • Ability to invade tissues.
  • Toxin production.
  • Immune evasion.
  • Resistance to antimicrobial drugs.
  • Ability to spread between hosts.
  • Host immune status.
  • Host genetics.
  • Nutritional status.
  • Age.
  • Pregnancy.
  • Comorbid disease.
  • Prior vaccination or immunity.

Virulence factors[edit]

Virulence factors are microbial traits that promote disease.

Host-pathogen interaction[edit]

Disease results from interaction between pathogen and host.

Host factors[edit]

Pathogen factors[edit]

  • Infectious dose.
  • Tissue tropism.
  • Replication rate.
  • Virulence factors.
  • Toxin production.
  • Immune evasion.
  • Antimicrobial resistance.
  • Environmental survival.
  • Transmission efficiency.

Environmental factors[edit]

  • Sanitation.
  • Clean water.
  • Food safety.
  • Crowding.
  • Climate.
  • Vector habitat.
  • Animal exposure.
  • Healthcare setting.
  • Travel.
  • War or displacement.
  • Poverty.
  • Occupational exposure.
  • Air quality.

Transmission[edit]

Pathogens spread through multiple routes.

Direct contact[edit]

Direct contact includes person-to-person spread through touching, kissing, sexual contact, bites, or contact with infected lesions.

Examples include:

Droplet transmission[edit]

Droplets are respiratory particles that travel short distances during coughing, sneezing, talking, or singing.

Examples include:

Airborne transmission[edit]

Airborne pathogens can remain suspended in air and travel farther.

Examples include:

Fecal-oral transmission[edit]

Pathogens spread through contaminated hands, water, food, or surfaces.

Examples include:

Bloodborne transmission[edit]

Bloodborne pathogens spread through blood exposure, injection drug use, needle sticks, transfusion, or certain sexual exposures.

Examples include:

Vector-borne transmission[edit]

Vectors such as mosquitoes, ticks, fleas, lice, and sandflies transmit pathogens.

Examples include:

Zoonotic transmission[edit]

Zoonotic diseases spread between animals and humans. CDC notes that zoonotic diseases can be caused by viruses, bacteria, parasites, and fungi.What Causes Parasitic Diseases(link). Centers for Disease Control and Prevention.

Examples include:

Vertical transmission[edit]

Vertical transmission occurs from parent to child during pregnancy, childbirth, or breastfeeding.

Examples include:

Reservoirs and carriers[edit]

A pathogen reservoir is the natural habitat where a pathogen lives, grows, and persists.

Reservoirs may include:

  • Humans.
  • Animals.
  • Birds.
  • Insects.
  • Soil.
  • Water.
  • Food.
  • Healthcare surfaces.
  • Biofilms.
  • Plants.

A carrier is an individual who harbors a pathogen and may spread it without showing symptoms. Carrier states are important in diseases such as typhoid fever, hepatitis B, Staphylococcus aureus colonization, and some respiratory infections.

Incubation, latency, and chronic infection[edit]

Examples:

Host defense mechanisms[edit]

The host uses multiple defense layers against pathogens.

Physical and chemical barriers[edit]

Innate immune system[edit]

The innate immune system responds rapidly and broadly.

Adaptive immune system[edit]

The adaptive immune system provides specific and memory-based responses.

Immune evasion[edit]

Pathogens have evolved many ways to evade host defenses. NCBI immunology resources describe immune evasion strategies such as antigenic variation, hiding inside host cells, resisting complement, and interfering with immune responses.Pathogens have evolved various means of evading or subverting normal host defenses(link). NCBI Bookshelf.

Common immune evasion strategies include:

  • Antigenic variation
  • Intracellular survival.
  • Capsule formation.
  • Biofilm formation.
  • Complement inhibition.
  • Resistance to phagocytosis.
  • Latency.
  • Suppression of interferon response.
  • Inhibition of antigen presentation.
  • Molecular mimicry.
  • Rapid mutation.
  • Immune checkpoint manipulation.
  • Destruction of immune cells.
  • Hiding in immune-privileged sites.

Pathogen detection and diagnosis[edit]

Diagnosis depends on pathogen type, body site, severity, and available testing.

Clinical evaluation[edit]

  • Medical history
  • Exposure history
  • Travel history
  • Animal exposure
  • Food and water exposure
  • Sexual history when relevant
  • Vaccination history
  • Immune status
  • Medication history
  • Physical examination
  • Vital signs
  • Assessment for sepsis or organ dysfunction

Laboratory methods[edit]

Imaging and pathology[edit]

Treatment[edit]

Treatment depends on the pathogen, infection site, host factors, severity, and resistance pattern.

Antibacterial therapy[edit]

Antibiotics are used for bacterial infections. Choice depends on likely pathogen, site of infection, local resistance, allergies, kidney and liver function, pregnancy status, and culture results.

Examples include:

Antiviral therapy[edit]

Antiviral drugs target specific viral infections.

Examples include:

Antifungal therapy[edit]

Antifungal drugs treat fungal infections.

Examples include:

Antiparasitic therapy[edit]

Antiparasitic drugs treat protozoal and helminth infections.

Examples include:

Prion disease management[edit]

There is no curative antimicrobial treatment for prion disease. Care is supportive and focuses on symptom control, safety, family counseling, infection-control precautions, and public health reporting when required.

Supportive care[edit]

Severe infections may require supportive care.

  • Fluid resuscitation.
  • Oxygen therapy.
  • Vasopressors.
  • Mechanical ventilation.
  • Dialysis.
  • Source control surgery or drainage.
  • Nutritional support.
  • Fever management.
  • Pain control.
  • Intensive care.
  • Rehabilitation after severe illness.

Antimicrobial resistance[edit]

Antimicrobial resistance occurs when pathogens evolve or acquire mechanisms that allow them to survive drugs designed to kill or inhibit them. It affects bacteria, viruses, fungi, and parasites, but antibiotic resistance in bacteria is one of the most urgent global threats.

WHO's 2025 global antibiotic resistance surveillance report found that one in six laboratory-confirmed bacterial infections causing common infections in people worldwide in 2023 were resistant to antibiotic treatments, and resistance rose in more than 40% of monitored pathogen-antibiotic combinations between 2018 and 2023.WHO warns of widespread resistance to common antibiotics worldwide(link). World Health Organization.Global antibiotic resistance surveillance report 2025(link). World Health Organization.

Important resistant pathogens include:

Strategies to reduce antimicrobial resistance include:

  • Antibiotic stewardship.
  • Vaccination.
  • Infection prevention.
  • Diagnostic testing.
  • Appropriate prescribing.
  • Completing treatment when instructed.
  • Avoiding antibiotics for viral infections.
  • Surveillance.
  • Sanitation.
  • Safe food production.
  • New drug development.
  • One Health approaches across humans, animals, and the environment.

Emerging and re-emerging pathogens[edit]

Emerging pathogens are new, newly recognized, or increasing infectious threats. Re-emerging pathogens are known pathogens that return or increase after decline.

Drivers include:

  • Global travel.
  • Urbanization.
  • Climate change.
  • Deforestation.
  • Wildlife trade.
  • Intensive agriculture.
  • Antimicrobial resistance.
  • Vaccine gaps.
  • War and displacement.
  • Laboratory detection improvements.
  • Changes in vector distribution.
  • Human-animal interface changes.

Examples include:

One Health[edit]

One Health recognizes that human health, animal health, plant health, and environmental health are interconnected. Many pathogens cross species boundaries or depend on environmental reservoirs.

One Health topics include:

  • Zoonotic diseases.
  • Antimicrobial resistance.
  • Food safety.
  • Vector control.
  • Wildlife disease surveillance.
  • Livestock health.
  • Environmental contamination.
  • Climate-sensitive infections.
  • Water and sanitation.
  • Global health security.

Public health and prevention[edit]

Pathogen control requires both individual and population-level prevention.

Personal prevention[edit]

  • Hand hygiene.
  • Safe food handling.
  • Clean water.
  • Vaccination.
  • Respiratory etiquette.
  • Staying home when contagious.
  • Safer sex.
  • Insect bite prevention.
  • Safe travel practices.
  • Avoiding contact with sick animals.
  • Wound care.
  • Not sharing needles.
  • Using antibiotics only when prescribed.

Healthcare infection prevention[edit]

  • Standard precautions.
  • Contact precautions.
  • Droplet precautions.
  • Airborne precautions.
  • Personal protective equipment.
  • Hand hygiene.
  • Sterilization.
  • Disinfection.
  • Isolation rooms.
  • Ventilation.
  • Needle safety.
  • Catheter care.
  • Surgical site infection prevention.
  • Antimicrobial stewardship.
  • Healthcare-associated infection surveillance.

Community prevention[edit]

  • Vaccination programs.
  • Sanitation.
  • Clean water systems.
  • Food inspection.
  • Vector control.
  • Outbreak surveillance.
  • Contact tracing.
  • Quarantine and isolation when indicated.
  • Public health education.
  • Disease reporting.
  • Infection prevention in schools, workplaces, and long-term care facilities.

Vaccination[edit]

Vaccination is one of the most effective tools for preventing infectious diseases. Vaccines train the adaptive immune system to recognize specific pathogens or their components and respond more rapidly after exposure.

Vaccine-preventable diseases include:

Pathogens and cancer[edit]

Some pathogens can cause or contribute to cancer.

Examples include:

Pathogens in plants and animals[edit]

Pathogens affect agriculture, ecosystems, food security, and veterinary medicine.

Plant pathogens[edit]

Plant pathogens include:

  • Plant viruses.
  • Bacteria.
  • Fungi.
  • Oomycetes.
  • Nematodes.
  • Viroids.

Examples include:

  • Phytophthora infestans
  • Wheat rust fungi.
  • Citrus greening disease.
  • Tobacco mosaic virus.
  • Xylella fastidiosa.
  • Fusarium wilt.

Animal pathogens[edit]

Animal pathogens affect pets, livestock, wildlife, and humans through zoonotic spillover.

Examples include:

  • Foot-and-mouth disease virus.
  • Avian influenza virus.
  • African swine fever virus.
  • Rabies virus.
  • Brucella species.
  • Leptospira species.
  • Salmonella species.
  • Toxoplasma gondii.

Beneficial microbes and pathobionts[edit]

Not all microbes are pathogens. Many microbes are beneficial or harmless.

  • Commensal microbiota help protect against pathogens.
  • Gut microbes support digestion and immune development.
  • Skin microbes can prevent colonization by harmful organisms.
  • Some microbes are used in food production and biotechnology.
  • Pathobionts are normally harmless organisms that can cause disease under certain conditions.
  • Disruption of the microbiome can increase risk of infection, such as Clostridioides difficile infection after antibiotics.

Pathogen research[edit]

Pathogen research supports diagnosis, treatment, prevention, vaccine development, and outbreak control.

Important fields include:

Biosafety and biosecurity[edit]

Work with pathogens requires safety systems to protect laboratory workers, the public, animals, and the environment.

Patient education[edit]

Patients and families should understand that infections differ by pathogen type, and treatment depends on the cause.

  • Antibiotics treat bacterial infections, not viral infections.
  • Some viral infections have antiviral treatments, but many require supportive care.
  • Fungal and parasitic infections require specific medications.
  • Vaccines prevent many serious pathogen-related diseases.
  • Hand hygiene reduces many infections.
  • Food safety prevents many gastrointestinal infections.
  • Insect bite prevention reduces vector-borne diseases.
  • Immunocompromised people may need special precautions.
  • Fever with confusion, stiff neck, rash, severe shortness of breath, chest pain, or low blood pressure requires urgent care.
  • Completing prescribed antimicrobial therapy helps prevent relapse and resistance.
  • Unnecessary antimicrobial use can contribute to resistance.

When to seek medical care[edit]

Seek medical care for:

  • Fever in an infant.
  • Fever with stiff neck.
  • Fever with confusion.
  • Fever with a purple or nonblanching rash.
  • Severe shortness of breath.
  • Chest pain.
  • Signs of sepsis.
  • Severe dehydration.
  • Persistent vomiting or diarrhea.
  • Blood in stool.
  • Severe abdominal pain.
  • Worsening skin infection.
  • Animal bite.
  • Tick bite with rash or fever.
  • Travel-related fever.
  • Immunocompromised status with fever.
  • Suspected meningitis.
  • Suspected tuberculosis.
  • Unexplained weight loss with fever or night sweats.
  • Infection not improving with treatment.

See also[edit]

Further reading[edit]

  • "The Pathogenic Potential of a Microbe".mSphere.2017;2(1)doi:10.1128/mSphere.00015-17.PMC:5326610.
  • "Principles of Infectious Diseases: Transmission, Diagnosis, Prevention, and Control".International Encyclopedia of Public Health.2017;
22-39.doi:10.1016/B978-0-12-803678-5.00516-6.PMC:7150340.
  • Infectious diseases - Symptoms and causes(link). Mayo Clinic.
  • Fungal Diseases(link). Centers for Disease Control and Prevention.
  • Parasites(link). Centers for Disease Control and Prevention.
  • About Prion Diseases(link). Centers for Disease Control and Prevention.
  • Bacterial Pathogenesis(link). NCBI Bookshelf.
  • Pathogens have evolved various means of evading or subverting normal host defenses(link). NCBI Bookshelf.
  • WHO warns of widespread resistance to common antibiotics worldwide(link). World Health Organization.
  • Global antibiotic resistance surveillance report 2025(link). World Health Organization.

External links[edit]

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