Microbiology || Infection and Disease || For Nursing Student ||

 

Infection and Disease

Definition and Types of Infections

Definition:
An infection occurs when microorganisms such as bacteria, viruses, fungi, or parasites invade a host's body, multiply, and cause harm. It may lead to diseases, characterized by specific clinical symptoms.


Types of Infections:

  1. Local Infection:
    Confined to a specific part of the body.

    • Example: Abscess, localized skin infection.
    • Symptoms: Redness, swelling, pain, and heat in the affected area.

  2. Systemic Infection:
    Spreads throughout the body, affecting multiple organs or tissues via the bloodstream.

    • Example: Sepsis, systemic fungal infection.
    • Symptoms: Fever, chills, fatigue, and organ dysfunction.

  3. Nosocomial Infection:
    Acquired in a hospital or healthcare setting.

    • Example: MRSA (Methicillin-resistant Staphylococcus aureus), urinary tract infections from catheters.
    • Cause: Poor hygiene, invasive procedures, or prolonged hospitalization.

  4. Opportunistic Infection:
    Occurs in individuals with weakened immune systems.

    • Example: Pneumocystis pneumonia in HIV/AIDS patients.
    • Cause: Microorganisms that are usually harmless in healthy individuals.

  5. Chronic Infection:
    Persists over a long period.

    • Example: Tuberculosis, Hepatitis B.
    • Symptoms: Gradual onset and prolonged progression.

  6. Acute Infection:
    Rapid onset and short duration.

    • Example: Influenza, common cold.
    • Symptoms: Fever, cough, and sore throat.

Pathogenesis of Microbial Infections

Pathogenesis refers to the mechanism by which an infection causes disease, involving complex interactions between the host and the pathogen. Key steps include:

  1. Entry into the Host:
    Pathogens enter the body through various routes:

    • Skin (cuts, wounds).
    • Mucous membranes (respiratory, gastrointestinal, urogenital tracts).
    • Bloodstream (injections, bites).

  2. Adherence to Host Tissues:
    Pathogens attach to host cells using adhesins or surface proteins.

    • Example: Pili in Escherichia coli for urinary tract infections.

  3. Invasion and Colonization:
    Pathogens invade host tissues, overcoming physical and immune barriers.

    • Enzymes like hyaluronidase degrade tissue barriers.

  4. Evasion of Immune Response:
    Microorganisms evade the immune system by:

    • Producing capsules (e.g., Streptococcus pneumoniae).
    • Altering surface antigens (e.g., Trypanosoma brucei).

  5. Toxin Production and Tissue Damage:

    • Exotoxins: Secreted proteins causing direct damage (e.g., botulinum toxin).
    • Endotoxins: Lipopolysaccharides in Gram-negative bacteria cause inflammation.

  6. Exit and Transmission:
    Pathogens leave the host to infect others through respiratory droplets, feces, or blood.

Factors Affecting Host-Pathogen Interactions

  1. Host Factors:

    • Immune Status: Immunocompromised individuals are more vulnerable.
    • Age: Infants and elderly have weaker immune defenses.
    • Nutrition: Malnutrition weakens immunity.
    • Genetics: Certain genetic traits (e.g., sickle cell anemia) affect susceptibility.

  2. Pathogen Factors:

    • Virulence: Ability to cause severe disease.
    • Dose: Higher microbial load increases infection likelihood.
    • Infectivity: Ability to establish infection.
    • Toxigenicity: Ability to produce toxins.

  3. Environmental Factors:

    • Hygiene practices.
    • Sanitation and clean water availability.
    • Climate and geography influencing pathogen survival.

  4. Behavioral Factors:

    • Lifestyle choices (e.g., unprotected sex, substance use).
    • Travel to endemic areas.

Common Infectious Diseases and Their Causative Agents

  1. Respiratory Infections:

    • Disease: Influenza

    • Agent: Influenza virus

    • Symptoms: Fever, cough, muscle aches.

    • Disease: Tuberculosis

    • Agent: Mycobacterium tuberculosis

    • Symptoms: Chronic cough, weight loss, night sweats.

  2. Gastrointestinal Infections:

    • Disease: Cholera

    • Agent: Vibrio cholerae

    • Symptoms: Watery diarrhea, dehydration.

    • Disease: Hepatitis A

    • Agent: Hepatitis A virus

    • Symptoms: Jaundice, nausea, fatigue.

  3. Vector-Borne Diseases:

    • Disease: Malaria

    • Agent: Plasmodium species (via Anopheles mosquito)

    • Symptoms: Fever, chills, sweating.

    • Disease: Dengue fever

    • Agent: Dengue virus (via Aedes mosquito)

    • Symptoms: High fever, severe headache, joint pain.

  4. Sexually Transmitted Infections (STIs):

    • Disease: HIV/AIDS

    • Agent: Human Immunodeficiency Virus (HIV)

    • Symptoms: Immunodeficiency, opportunistic infections.

    • Disease: Syphilis

    • Agent: Treponema pallidum

    • Symptoms: Sores, rash, organ damage in late stages.

  5. Nosocomial Infections:

    • Disease: MRSA infection

    • Agent: Methicillin-resistant Staphylococcus aureus

    • Symptoms: Skin infections, sepsis.

    • Disease: Catheter-associated UTI

    • Agent: Escherichia coli, Klebsiella pneumoniae

    • Symptoms: Painful urination, fever.

  6. Fungal Infections:

    • Disease: Candidiasis

    • Agent: Candida albicans

    • Symptoms: White patches (oral thrush), genital itching.

    • Disease: Aspergillosis

    • Agent: Aspergillus species

    • Symptoms: Respiratory distress, fever.

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