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Chain of Infection. Dr. Mohamed El Bashaar. Causative Agent. Susceptible Host. Reservoir. Portal of Exit. Portal of Entry. Mode of Transmission. CAUSATIVE AGENTS:. A biologic agent/s capable of causing infectious diseases. Bacteria,Parasites , Fungi & Viruses. Reservoir.

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Chain of infection

Chain of Infection

Dr. Mohamed El Bashaar


Causative Agent

Susceptible Host


Portal of Exit

Portal of Entry

Mode of Transmission


Causative agents

  • A biologic agent/s capable of causing infectious diseases.

    Bacteria,Parasites, Fungi & Viruses



  • Place in which an infectious agent can survive but may or may not multiply

    • Salmonella in milk: survives and multiplies

    • Hepatitis B virus on surface of hemodialysis machine: survives but does not multiply.


Two major types of human reservoir
Two Major Types of Human Reservoir

  • Cases(Patients)

    *Acute clinical

    * Subclinical

  • Carrier (HBV,HIV)


Portal of exit associated w human reservoirs
Portal of Exit Associated w/ Human Reservoirs

  • Respiratory Tract

  • Genitourinary Tract

  • Gastrointestinal Tract

  • Skin/Mucous Membrane

  • Transplacental (mother to fetus)

  • Blood


Modes of transmission
Modes of Transmission

  • The mechanism for transfer of an infectious agent from a reservoir to a susceptible host.


Means of transmission five main routes
Means of TransmissionFive Main Routes

  • Droplet

  • Airborne

  • Common Vehicle (Food, blood)

  • Vector-borne

  • Contact

    • Direct Contact

    • Indirect Contact (Objects)


Portal of entry
Portal of Entry

  • The path by which an infectious agent enters the susceptible host

    • Respiratory tract

    • Genitourinary tract

    • Gastrointestinal tract

    • Skin/mucous membrane

    • Transplacental (fetus from mother)

    • Parenteral (percutaneous, via blood)


Susceptible host
Susceptible Host

  • A person usually lacking effective resistance to a particular pathogenic agent.


Sources and modes of transmission of infection
Sources and modes of transmission of infection


Person-to-Person Spread

Skin-to-skin contact, clothes, (folliculitis with Staphylococci or MRSA ,viral as HSV1)

Droplets during coughing, sneezing (common cold, flu, swine flu, pneumonia, bacterial meningitis), or kissing (infectious mononucleosis, cold sores)

Stool-to-mouth (fecal-oral) spread, usually via dirty hands or utensils (hepatitis A, Giardia, pinworms(oxyuris), cholera, poliomyelitis)


  • Sexually transmitted (gonorrhea, Chlamydia, genital HSV2, genital warts (human papillomavirus – HPV), AIDS & syphilis)

  • Blood-to-blood contact by:

    • contaminated needles, usually by drug addicts or health workers (hepatitis B, C, AIDS)

    • blood transfusion (hepatitis B,C, AIDS, viral hemorrhagic fevers)

    • Vectors: mosquitoes (Malaria, Dengue fever)


Spread from mother to fetus during pregnancy (hepatitis B, C, HIV, HSV-1, HSV-2, rubella, toxoplasma, varicella, syphilis, bird flu), or delivery (Chlamydia trachomatis, Neisseriagonorrheae, group B Streptococci=S.agalactiae)

An autoinfection – the spread of an infection from one body part to another, usually by hands or clothes (folliculitis, impetigo).


2. Waterborne Infections

By drinking contaminated water, one can contract, cholera, dysentery, typhoid fever, amebiasis & cryptosporidium . Tap water in hospitals may contain Legionella.

By swimming in contaminated swimming or spa pools, or lakes, intestinal parasite Cryptosporidium, eye and middle ear infections. Certain parasites may enter through the skin(schistosoma).

Water in public showers may hold Legionella;

Flood water may contain various pathogenic microbes


3. Airborne Infections

Industrial cooling or hot water systems, air condition can be a source of M. tuberculosis or Legionella

4. Soil

During walking barefoot, Clostridium tetani, or intestinal parasites, like Strongyloidesstercoralis or hookworms(Ancylostoma) can be contacted.

Eating with soil-contaminated hands can also result in infection by parasites.


5. Food-Borne Infections

Food poisoningis an infection of the gastrointestinal tract caused by microbes from contaminated food: bacteria like Salmonella or E.coli, toxins from Staphylococcus aureus or Clostridium botulinum or viruses like Enterovirus.



6. NosocomialInfections : Health-care acquired infections(HAI):

Usually caused by bacteria that are resistant to antibiotics (multi-drug resistant strains).

Urinary tract infections from urinary catheters (CA-UTI) .

Surgical-site infections (SSI).

Catheter related Blood stream infections(CR-BSI).

Ventilator associated pneumonia(VAP)

Fungal infections, in patients with low immunity.