Brief History of Infectious Disease

What is parasitism?

One organism benefits; the host is harmed.

What is commensalism?

One organism benefits; the host is unaffected.

What is mutualism?

Both organisms benefit

What is the endosymbiotic theory?

Some organelles (e.g., mitochondria) evolved from microbes living inside host cells.

What is infection?

Establishment of a microorganism in a host; often short-lived, asymptomatic, and sometimes beneficial.

What is an infectious disease?

An infection that causes damage and produces clinical symptoms.

What is a pathogen?

A microorganism capable of causing disease.

Difference between principal and opportunistic pathogens?

Principal pathogens cause disease in healthy hosts; opportunistic pathogens exploit weakened hosts.

What allows a pathogen to cause disease?

Ability to breach host defenses, survive, replicate, evade immunity, damage tissue, and transmit.

What is virulence?

The degree to which a pathogen causes disease.

What are virulence factors?

Genes or products that increase pathogenicity (e.g., toxins, adhesion, immune evasion).

Give examples of immune evasion:

H. influenzae destroys IgA
• T. pallidum coats itself to hide
• Mycobacterium suppresses immunity

What is pathogenic evolution?

Pathogens adapt to specific environments or hosts over time.

What is a host-restricted (obligate) pathogen?

A pathogen that survives only within a specific host.

What do pseudogenes indicate?

Long-term adaptation to a stable environment or host.

What are plasmids?

Circular DNA carrying antibiotic resistance and virulence genes.

What types of organisms can be pathogens?

Viruses, bacteria, and eukaryotic parasites.

What are ancestral diseases?

Endemic diseases with reservoirs or chronic infection; not population-dependent.

What are diseases of civilization?

Epidemic diseases requiring large populations; often evolved from animal diseases.

What is zoonotic disease?

A disease transmitted from animals to humans.

Examples of zoonotic origins?

Measles (cattle), TB (cattle), influenza (swine/birds), malaria (birds).

What defines pathogen “success”?

Ability to infect, reproduce, and transmit.

What defines host “success”?

Genetic defenses that reduce disease risk (e.g., hemoglobinopathies).

Key takeaway on infection and disease?

Disease results from complex interactions between host and pathogen.

Big-picture summary?

Human diseases evolved with us, and their severity depends as much on the host as the pathogen.