Host defense strategies
1. Avoidance
2. Elimination/resistance
3. Tolerance
What are the three major categories of host defenses against parasites?
A regulated elevation of core body temperature induced by pyrogens, which functions to inhibit parasite replication and boost immune function
What is a fever?
They are diseases representing a new threat to human populations, including previously unknown diseases (spillovers), re-emerging diseases that were once controlled, and the evolution of new resistant strains of known pathogens
What are Emerging Infectious Diseases (EIDs)?
A strategy used to control resistance involving more judicious prescription of antibiotics, ensuring patients complete full courses, and limiting antibiotic use in agriculture to reduce selective pressure
What is "antibiotic stewardship"?
The CCR5-delta 32 deletion, which prevents the virus from entering immune cells
What genetic adaptation found in Northern Europeans provides resistance to HIV?
A suite of psychological mechanisms, primarily involving the emotion of disgust, motivates individuals to avoid potential sources of infection (such as feces, spoiled food, or individuals who are sick).
What is the "behavioral immune system"?
They are adaptive motivational states—including lethargy, loss of appetite, and social withdrawal—that facilitate recovery from infection by conserving energy and prioritizing immune function
What are "sickness behaviors"?
A frequent, ongoing contact and transmission of viruses between wildlife and humans (often through hunting or habitat encroachment) that provides opportunities for pathogens to spill over and potentially adapt to humans
What is "viral chatter"?
Simultaneous use of multiple drugs with different mechanisms, which makes resistance evolution statistically unlikely because a parasite would need to spontaneously develop multiple specific mutations at once
What is "combination drug therapy"?
Political instability led to the collapse of health services and vaccination programs, causing a massive re-emergence of the disease
What role did the collapse of the Soviet Union contribute to a diphtheria epidemic?
The evolutionary concept that the immune system is selected to over-react to potential threats (causing false alarms like allergies) because the cost of a false alarm is low compared to the catastrophic cost of failing to detect a lethal infection
What is the "smoke detector principle"?
Innate immunity is a generalized, fast-acting first line of defense (including skin and inflammation), whereas adaptive immunity is a slower, specific response that develops memory (B-cells and T-cells) to target specific pathogens
What is the difference between innate and adaptive immunity?
An ecological phenomenon where high biodiversity reduces disease risk because incompetent reservoir species "dilute" the prevalence of the pathogen; human disturbance often removes these species, leaving behind competent reservoirs
What is the "dilution effect"?
An ecological approach to treatment where drug-sensitive or beneficial bacteria are maintained in the body to compete with and suppress the growth of resistant, harmful bacteria
What is "competitive suppression"?
She was an asymptomatic carrier of Salmonella typhi, meaning she appeared healthy but shed bacteria, infecting people through her cooking.
What is "Typhoid Mary", and how was she a public health danger?
A risky strategy where the host activates defenses that damage its own tissues (such as high fever or nutrient withholding) to harm the parasite even more, effectively betting that the host can withstand the damage longer than the parasite
What is "immune brinksmanship"?
It is a genetic mutation found in Northern European populations consisting of a 32-nucleotide deletion in a cell surface receptor; in homozygous individuals, it prevents HIV from entering cells, conferring resistance
What is the CCR5-delta 32 deletion?
Bats live in dense aggregations, have high species diversity, and possess unique immune systems—possibly evolved to handle the metabolic stress of flight—that allow them to tolerate high viral loads
What are the biological factors that make bats frequent sources of EIDs?
An evolutionary trade-off where a bacterium evolving resistance to one drug inadvertently becomes more sensitive to a second drug, allowing clinicians to cycle treatments to eliminate the infection
What is "collateral sensitivity"?
Researchers discovered that HIV spilled over from chimpanzees much earlier than the 1980s—likely around 1900—and had been spreading in humans for decades before being recognized.
What did researchers discover about the origin of HIV (using phylogenetics and "molecular clocks"?
A model showing that natural selection optimizes the total cost; in some cases, the optimal strategy involves a defense so expensive (e.g., severe inflammation) that the cost of the defense itself exceeds the direct cost of the disease, because this investment stops the infection quickly
What is the "optimization of defense" regarding the cost of disease versus the cost of defense?
The process by which the adaptive immune system generates a vast diversity of B- and T-cells and then selects the specific variants that best match a new microbial threat to multiply, allowing the host to adapt to fast-evolving microbes within a single lifespan
What is "somatic natural selection" within the immune system?
Large urban populations provide a "critical community size" that produces enough new susceptible individuals (babies) to maintain transmission chains of diseases (like measles) that would otherwise burn out in smaller populations
What is the relationship between urbanization and the maintenance of "crowd diseases"?
The principle that selective pressures decline with age; for malaria control, this involves using slow-acting insecticides that kill mosquitoes only after they have reproduced (reducing resistance evolution) but before the malaria parasite inside them becomes infectious
What is the "selection shadow" in the context of vector control?
The 2014–2016 Ebola epidemic demonstrates that urbanization can greatly increase the spread of infectious disease. Unlike earlier outbreaks confined to isolated rural villages, this epidemic reached densely populated urban slums, where high population density and increased human mobility amplified transmission and made containment far more difficult than in rural settings.
What is the impact of urbanization on infectious disease as illustrated by the 2014–2016 Ebola epidemic?