Pathophysiology
Mortality and morbidity rates for infectious diseases vary greatly by geographical location. World Health Organization (WHO) statistics provide striking documentation that deaths due to infectious diseases are much higher in poor countries than in the countries with the highest income levels. It is particularly striking that malaria and tuberculosis are significant causes of death in low-resource countries, whereas these diseases are rarely fatal in wealthy countries. Other infectious diseases that significantly cause death in low-resource countries are pneumonia, diarrhea, and HIV infection. The top ten causes of death in lower and middle-resource countries In lower and middle-resource countries also include lower respiratory infections, tuberculosis, and diarrheal diseases.
1. Define infection and identify the differences between colonization and infection, giving specific examples of each.
2. How does infection start, and what is the usual course?
3. What factors influence the course of an infection? Give specific examples of different trajectories of infection.
Sample Solution
Infection vs. Colonization:
Infection:
- Defined as the invasion and multiplication of micro-organisms (bacteria, viruses, fungi, parasites) within a host, causing harm to the host tissue and function.
- Requires the microbe to establish itself, overcome host defenses, and cause damage.
- Symptoms like fever, inflammation, and tissue damage are common.
- Examples:
- Pneumonia: Streptococcus pneumoniae bacteria causing lung infection.
- Urinary tract infection: Escherichia coli bacteria colonizing the urethra and bladder.
Full Answer Section
Colonization:
- Presence and multiplication of micro-organisms on or within a host without causing harm.
- Microbes don't overcome host defenses or damage tissues.
- May be normal flora (e.g., gut bacteria) or transient (e.g., bacteria on skin).
- No symptoms unless the colonized area changes or host defenses weaken.
- Examples:
- Skin flora: Staphylococcus epidermidis bacteria living on healthy skin.
- Mouth flora: Streptococcus mitis bacteria colonizing the mouth without causing harm.
Key Differences:
- Host response: Infection triggers an immune response to fight the microbe, while colonization doesn't.
- Harm to host: Infection causes damage and symptoms, while colonization doesn't.
- Location: Infection typically involves deeper tissue invasion, while colonization often occurs on surfaces.
2. Start and Course of Infection:
Initiation:
- Infection starts when a microorganism encounters a susceptible host and breaches a barrier like skin, mucous membranes, or intestinal lining.
- Factors influencing initiation include:
- Infectivity and virulence of the microbe.
- Host immune status and presence of defense mechanisms.
- Environmental factors like hygiene, sanitation, and climate.
Typical Course:
- Incubation: Microbe replicates silently, no symptoms.
- Early stage: Immune response activated, mild symptoms may appear.
- Acute stage: Symptoms peak, host fights microbes fiercely.
- Convalescence: Symptoms decline, immune system eliminates or controls microbes.
- Resolution: Complete recovery or chronic infection in some cases.
Note: Course can vary depending on the microbe, host factors, and treatment. Some infections may be rapid and fulminant, while others progress slowly.
3. Factors Influencing Infection Course:
Microbe factors:
- Virulence: Ability to cause damage and invade tissues.
- Mode of transmission: How the microbe spreads and its entry point.
- Communicability: Ease of transmission from host to host.
Host factors:
- Age: Infants and elderly have weaker immune systems.
- Immune status: HIV/AIDS or chronic diseases weaken immunity.
- Genetic predispositions: Some individuals are more susceptible to certain infections.
- Nutrition and overall health: Malnutrition and poor health worsen resistance.
Environmental factors:
- Sanitation and hygiene: Poor sanitation facilitates microbial spread.
- Climate and geography: Certain regions have higher prevalence of specific infections.
- Socioeconomic conditions: Poverty and overcrowding increase exposure to infection.
Trajectory Examples:
- Mild cold: Virus causes mild symptoms, resolves within a week.
- Sepsis: Bacterial infection triggers overwhelming immune response, potentially fatal.
- Tuberculosis: Chronic bacterial infection, with potential for reactivation years later.
- HIV/AIDS: Virus weakens immune system, leading to vulnerability to other infections.
Understanding these factors and their role in the course of infection is crucial for developing effective prevention strategies, diagnostics, and treatment options for various infectious diseases.