Infection Basics: Symbiosis, Pathogenesis and Transmission

Commensalism: One species benefits, while the other is neither helped nor harmed (e.g., microbes living on skin secretions, ear, or external genitals).

Mutualism: Both organisms benefit (e.g., E. coli in the large intestine synthesizing Vitamin K and B/G vitamins).

Symbiosis: Two species living together for mutual benefit (e.g., normal microbiota and host).

Parasitism: One organism (parasite) benefits at the host's expense; includes all disease-producing pathogens.

Opportunistic Microbes: Organisms (resident or transient) that ordinarily do not cause disease, but take advantage of compromised host defenses, failure of immune barriers, or normal flora disturbances.

Resident Microbiota: Microbes that become permanently established in body sites in contact with the outside environment.

Transient Microbiota: Microbes that occupy the body for only short periods.

Microbial Antagonism: Normal flora protecting the host by preventing the overgrowth of harmful pathogens.

Anatomical Sites Harboring Normal Flora: Skin, upper respiratory tract (oral cavity, pharynx, nasal mucosa), GI tract (mouth, colon, rectum, anus), outer urethra, external genitalia, vagina, external ear, and external eye.

Sterile (Microbe-Free) Sites & Fluids: All internal organs/tissues: Heart, liver, kidneys, bladder, lungs, brain, spinal cord, bones, ovaries/testes, glands, sinuses, middle/inner ear, muscles.

Internal fluids: Blood, CSF, urine inside kidneys/ureters/bladder, saliva prior to entering the oral cavity, semen prior to entering the urethra.

Finding a Portal of Entry: Microbes enter via skin (cuts/abrasions), GI tract (ingestion of food/drink), respiratory tract (inhalation—most common entry route), urogenital tract (sexual contact), or transplacental transmission.

Attaching Firmly (Adhesion): Microbes gain a foothold using specialized structures:

Fimbriae / Pili: N. gonorrhoeae (Type IV pili), E. coli, Shigella, Salmonella.

Glycocalyx / Slime Layer: V. cholerae, S. mutans (dextran slime on teeth).

Hooks / Specialized Tips: Treponema (tapered hook), Mycoplasma (specialized tip).

Viral Spikes / Capsids: Influenza, HIV, Poliovirus.

Suction Discs / Flagella: Giardia lamblia (suction disc), Trypanosoma.

Surviving Host Defenses: Avoiding phagocytosis, avoiding destruction inside phagocytes, or evading immune detection.

Causing Damage & Disease: Direct damage: Production of exoenzymes, toxins, or cell lysis. & Indirect damage: Triggering an inappropriate or excessive host immune response.

Exiting the Host (Portals of Exit): Depart via respiratory/salivary secretions, skin cells, feces, urogenital discharge, or blood.

Incubation Period: Time between initial contact and appearance of first symptoms; microbe is multiplying without visible damage.

Prodromal Stage: Short period of vague, non-specific discomfort.

Period of Invasion (Acme): Pathogen multiplies rapidly and reaches peak intensity with specific signs and symptoms.

Convalescent Period: Immune system responds, symptoms decline, and tissues repair.

Sign: Objective evidence observed or measured by an examiner (e.g., fever, cell counts).

Symptom: Subjective evidence felt only by the patient (e.g., pain, nausea).

Syndrome: A specific combination of signs and symptoms occurring together.

Sequelae: Long-term or permanent damage left after recovery.

Infection Patterns:

Primary vs. Secondary: Initial infection vs. a subsequent infection caused by a different opportunistic microbe.

Superinfection: Secondary infection resulting from destruction of normal flora by broad-spectrum antibiotics.

Local vs. Systemic vs. Focal: Confined to one site vs. spread throughout body fluids/blood vs. local infection that breaks loose to infect other tissues.

Mixed (Polymicrobial): Caused by multiple microbes working simultaneously.

Blood Conditions: Leukocytosis: Increased WBC count. Leukopenia: Decreased WBC count. Septicemia: Pathogens actively multiplying in large numbers in the blood. Bacteremia / Viremia: Small numbers of bacteria or viruses present in blood, not necessarily multiplying.

Reservoir: Primary natural habitat from which a pathogen originates.

Source: Individual or object from which an infection is actually acquired.

Zoonosis: Disease indigenous to animals that can be transmitted to humans.

Epidemiological Patterns:

Endemic: Infectious disease continually present in a population.

Epidemic: Sudden spike in incidence higher than normal expectations.

Pandemic: Epidemic spreading worldwide.

Sporadic: Random, unpredictable isolated cases.

Transmission Modes:

Direct: Person-to-person body contact, direct fecal-oral, vertical (mother to fetus).

Indirect (Vehicle): Inanimate carriers such as fomites (non-living objects), water, food, air/aerosols.

Biological Vector: Animal/insect actively participating in pathogen life cycle or multiplication.

Mechanical Vector: Animal/insect passively transporting pathogens on external body parts without being infected.

HAIs (Nosocomial Infections): Infections acquired during hospital stays or medical care.

Top 4 Nosocomial Pathogens: E. coli, Staphylococcus aureus, Enterococci, and Pseudomonas.

Universal Precautions: Guidelines based on treating all patient specimens as potentially infectious to prevent healthcare transmission.