Principles of Disease and Epidemiology

Pathology, Infection, and Disease

Understanding disease in microbiology involves several key concepts that describe the cause, development, and effects of disease in the host.

• Pathology: The study of disease, including its causes and effects.

• Etiology: The cause of a disease, often determined by identifying the responsible pathogen.

• Pathogenesis: The process by which disease develops in the host.

• Infection: The invasion or colonization of the body by pathogenic microorganisms.

• Disease: An abnormal state in which the body is not functioning normally due to infection or other factors.

The Human Microbiome

The human microbiome consists of the collection of microorganisms that inhabit various regions of the body. These microbes play essential roles in health and disease.

• Establishment: Begins in utero and expands through exposure to food, people, and pets.

• Normal microbiota: Permanently colonize the host and typically do not cause disease under normal conditions.

• Transient microbiota: Present temporarily, for days, weeks, or months.

• Distribution and composition: Determined by nutrients, physical and chemical factors, host defenses, and mechanical factors.

Representative Normal Microbiota by Body Region

Different regions of the body host distinct populations of normal microbiota, which are adapted to their local environments.

Example: Vaginal birth results in a microbiome dominated by Lactobacillus and Bacteroides, while Cesarean birth leads to a microbiome resembling human skin, including Staphylococcus aureus.

Relationships Between the Normal Microbiota and the Host

Normal microbiota interact with the host in various ways, influencing health and disease.

• Microbial antagonism (competitive exclusion): Competition between microbes for nutrients and space.

• Protection: Normal microbiota protect the host by competing for nutrients, producing substances harmful to invaders, and affecting pH and oxygen levels.

• Symbiosis: Relationship between microbiota and host, classified as:

  • Commensalism: One organism benefits, the other is unaffected.

  • Mutualism: Both organisms benefit.

  • Parasitism: One organism benefits at the expense of the other.

• Opportunistic pathogens: Some normal microbiota can cause disease under certain conditions.

The Etiology of Infectious Diseases

Determining the cause of infectious diseases is fundamental in microbiology. Koch’s postulates provide a systematic approach.

• Koch’s Postulates:

  1. The same pathogen must be present in every case of the disease.

  2. The pathogen must be isolated from the diseased host and grown in pure culture.

  3. The pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal.

  4. The pathogen must be isolated from the inoculated animal and shown to be the original organism.

• Exceptions: Some pathogens cause multiple diseases, only infect humans, or cannot be cultured.

Classifying Infectious Diseases

Diseases are classified based on their symptoms, signs, and modes of transmission.

• Symptoms: Subjective changes felt by the patient.

• Signs: Objective changes measurable or observable.

• Syndrome: Specific group of signs and symptoms accompanying a disease.

• Communicable disease: Spread from one host to another.

• Contagious disease: Easily and rapidly spread.

• Noncommunicable disease: Not spread between hosts.

Occurrence of Disease

Tracking disease occurrence is essential for epidemiology.

• Incidence: Number of new cases during a specific time period.

• Prevalence: Total number of cases at a specified time, including old and new cases.

• Sporadic disease: Occurs occasionally.

• Endemic disease: Constantly present in a population.

• Epidemic disease: Acquired by many people in a short time.

• Pandemic disease: Worldwide epidemic.

Severity or Duration of Disease

Diseases vary in their severity and duration.

• Acute disease: Rapid onset, short duration.

• Chronic disease: Slow development, long duration.

• Subacute disease: Intermediate between acute and chronic.

• Latent disease: Inactive for a time, then activates.

• Herd immunity: Immunity in most of a population.

Extent of Host Involvement

Infections can be localized or spread throughout the body.

• Local infection: Limited to a small area.

• Systemic infection: Throughout the body.

• Focal infection: Systemic infection that began locally.

• Sepsis: Toxic inflammatory condition from spread of microbes or toxins.

• Bacteremia: Bacteria in the blood.

• Septicemia: Growth of bacteria in the blood (blood poisoning).

• Toxemia: Toxins in the blood.

• Viremia: Viruses in the blood.

• Primary infection: Initial acute infection.

• Secondary infection: Opportunistic infection after primary infection.

• Subclinical disease: No noticeable signs or symptoms.

Predisposing Factors

Various factors can make the body more susceptible to disease.

• Gender

• Inherited traits (genes)

• Lack of vaccination

• Fatigue

• Age

• Lifestyle

• Nutrition

• Chemotherapy

Development of Disease

The progression of disease follows distinct stages.

• Incubation period: Interval between initial infection and first signs/symptoms.

• Prodromal period: Early, mild symptoms.

• Period of illness: Disease is most severe.

• Period of decline: Signs and symptoms subside.

• Period of convalescence: Body returns to prediseased state.

The Spread of Infection

Understanding how infections spread is crucial for prevention and control.

Reservoirs of Infection

• Human reservoirs: Carriers may have inapparent or latent infections.

• Animal reservoirs: Zoonoses are diseases transmitted from animals to humans.

• Nonliving reservoirs: Soil and water.

Transmission of Disease

• Contact transmission:

  • Direct contact: Close association between infected and susceptible host.

  • Congenital transmission: Mother to fetus/newborn.

  • Indirect contact: Via fomites (nonliving objects).

  • Droplet transmission: Airborne droplets less than 1 meter.

• Vehicle transmission: Transmission by inanimate reservoirs (air, water, food).

• Vector transmission: Arthropods (fleas, ticks, mosquitoes) transmit disease by:

  • Mechanical transmission: Pathogen carried on feet.

  • Biological transmission: Pathogen reproduces in vector, transmitted via bites or feces.

Healthcare-Associated Infections (HAIs)

HAIs, also known as nosocomial infections, are acquired during treatment in healthcare facilities.

• Affect 1 in 25 hospital patients; 2 million per year infected, over 70,000 deaths.

• Result from microorganisms in hospital environment, weakened host, and chain of transmission.

• Compromised host: Resistance to infection is impaired.

Principal Sites of Healthcare-Associated Infections

Control of Healthcare-Associated Infections

• Universal precautions: Basic minimum practices.

• Transmission-based precautions: Supplemental for known/suspected infections.

• Reduce pathogens: Handwashing, disinfecting, cleaning instruments, using disposables, infection control committees.

Emerging Infectious Diseases

Emerging infectious diseases are new, increasing in incidence, or have potential to increase soon. Most are zoonotic, viral, and vector-borne.

• Contributing factors: Genetic recombination, evolution of new strains, widespread antibiotic/pesticide use, antibiotic resistance, weather changes, modern transportation, insect vectors.

Epidemiology

Epidemiology is the study of where and when diseases occur and how they are transmitted in populations.

• Determine etiology of disease

• Identify factors concerning spread

• Develop methods for control

• Assemble data and graphs to outline incidence

The Centers for Disease Control and Prevention (CDC)

• Collects and analyzes epidemiological information in the U.S.

• Publishes Morbidity and Mortality Weekly Report (MMWR)

• Morbidity: Incidence of specific notifiable disease

• Mortality: Deaths from notifiable diseases

• Morbidity rate: Number affected in relation to population in a given time

• Mortality rate: Number of deaths in relation to population in a given time