Chapter 14: Principles of Disease and Epidemiology

Chapter 14: Principles of Disease and Epidemiology

Pathology, Infection, and Disease

Pathology: Study of disease.
Etiology: Study of the cause of a disease.
Pathogenesis: Development of disease.
Infection: Invasion or colonization of the body by pathogens.
Disease: Abnormal state of the body where normal functions are disrupted.

Types of Diseases

Infectious Disease: Caused by pathogens like bacteria and viruses.
Non-infectious Disease: Not caused by pathogens.

Normal Microbiota

Resident microbiota: Always present in/on the human host.
Transient microbiota: Temporary presence; last for hours to months.

Microbe and Host Relationships

Symbiosis: Interaction between microbe and host.
Mutualism: Both benefit; e.g., E. coli in the colon produces Vitamin K.
Parasitism: One benefits at the other's expense; e.g., pathogenic bacteria.

Koch’s Postulates

Four criteria to establish a causative relationship between a microbe and a disease:

The pathogen must be present in every case of the disease.
The pathogen must be isolated from the diseased host and grown in pure culture.
The pathogen must cause the disease in a healthy host after inoculation.
The pathogen must be isolated from the inoculated animal.

Exceptions:
Some pathogens can cause various diseases.
Some pathogens only infect humans.
Some microbes cannot be cultured artificially.

Classifying Infectious Diseases

Symptoms: Changes in body function felt by the patient.
Signs: Observable or measurable changes.
Syndrome: A group of specific signs & symptoms associated with a disease.
Communicable disease: Spreads from one host to another (e.g., Chickenpox).
Noncommunicable disease: Not spread between hosts (e.g., Tetanus).

Disease Occurrence

Incidence: Number of new cases during a specific time.
Prevalence: Total number of cases at a given time, including old & new.
Sporadic: Occasional occurrences of the disease.
Endemic: Disease always present in a population.
Epidemic: Rapid increase of disease in a specific area.
Pandemic: Worldwide epidemic.

Severity or Duration of a Disease

Acute: Rapid onset and short duration.
Chronic: Develops slowly and lasts a long time.
Subacute: Intermediate between acute and chronic.
Latent: Inactive phase that may reactivate.

Herd Immunity

Population immunity achieved through vaccinations.

Extent of Host Involvement

Local infection: Limited to a small area.
Systemic infection: Affects the entire body.
Sepsis/Septicemia: Uncontrolled spread of microbes in the blood.
Primary infection: Initial infection leading to illness.
Secondary infection: Opportunistic infection following a primary infection.

Predisposing Factors

Factors that increase susceptibility to disease:
Gender, Age, Lifestyle, Inherited traits, Climate, Poor sanitation.

Development/Stages of Disease

Incubation period: No signs present.
Prodromal period: Early mild symptoms.
Period of illness: Most severe signs and symptoms.
Peak of illness: Leads to decline or death.
Period of decline: Symptoms subside.
Convalescence: Body returns to pre-diseased state.

Reservoirs of Infection

Human: Sick individuals or carriers.
Animal: Zoonoses capable of transmission.
Non-living: Contaminated objects or environments.

Transmission of Diseases

Contact Transmission:
Direct, Indirect (fomites), Droplet (airborne particles).
Vehicle Transmission: Inanimate reservoirs (airborne, waterborne, foodborne).
Vector Transmission: Via arthropods; mechanical and biological methods.

Healthcare-Associated Infections (HAIs)

Infections acquired in healthcare settings; 1 in 25 hospital patients affected.
Common sources include improperly cleaned environments and weakened immune systems.

Epidemiology

Study of the occurrence and transmission of diseases in populations.
The CDC plays a key role in tracking morbidity and mortality related to diseases.

Nationally Notifiable Infectious Diseases

Must be reported to the CDC, including Hepatitis, HIV, Influenza, and more.