Host Population Regulation, Disease Virulence, and Transmission Heterogeneity

Vocabulary flashcards covering ecological and epidemiological concepts from the lecture notes, including host population regulation, virulence, transmission types, stochasticity, and individual heterogeneity.

Rinderpest

A pathogen that was eliminated in the Serengeti by vaccinating border cattle, which caused the wild wildebeest population to skyrocket from 200,000 to 1.2 million (an 8-fold increase) without directly vaccinating any wildebeest.

Canine distemper virus

A pathogen linked to sudden population crashes in Serengeti lions after the lion population became denser and exceeded a critical population size.

Handling time

The period required for a predator to become full and digest food, which causes predator consumption rates to satiate, unlike insatiable pathogens with fast reproductive rates.

Population regulation

The leveling off of a population when births equal deaths at equilibrium, which requires density-dependent factors such as increased disease mortality at higher densities.

Intermediate virulence hypothesis

The principle that a disease regulates host population size only at intermediate levels of mortality, because very high virulence shortens the infectious period and drives $R_0$ below 1, while very low virulence causes too few deaths to regulate the population.

Sterilizing diseases

Pathogens that sterilize infected hosts without shortening their infectious period, exerting a more significant regulating impact on host populations than lethal pathogens.

Critical community size

The threshold host density calculated by setting $R_0 = 1$, below which an ordinary infectious disease cannot maintain contacts and dies out.

Ordinary infectious diseases (OID)

Diseases where contact rates drop continuously as host density falls, meaning small or endangered populations do not sustain the pathogen and are typically not at risk of disease-induced extinction.

Frequency-dependent transmission

A transmission mode common in sexually transmitted diseases where contact rates remain high even as host density drops, posing an extinction risk to small host populations such as koalas infected with chlamydia.

Contagious facial tumors

An infectious cancer cell line transmitted between Tasmanian devils during aggressive fighting behavior that persists even at low population densities.

Shared disease

A pathogen that circulates in a large reservoir host, creating a continuous risk of infection and potential extinction for a secondary declining host population.

Stochastic models

Mathematical disease models that incorporate chance events and randomness, demonstrating that epidemics can still fail to establish even when $R_0 > 1$.

$$R_0$$

The basic reproduction number calculated for a typical infectious individual, which defines the average disease growth rate, probability of fade-out, and required control measures.

Super-spreader

An infected individual who transmits a pathogen to significantly more secondary cases than the average person, serving as a central focus in transmission networks.

$$z$$

The actual number of secondary cases caused by a particular infected individual in an outbreak.

Dispersion parameter ($k$)

A parameter measuring individual variation in disease transmission; a low $k$ value indicates extreme individual heterogeneity with super-spreaders, whereas a high value indicates that individuals are intrinsically similar.

20-80 rule

The empirical pattern observed in heterogeneous diseases such as SARS where approximately 20% to 25% of infected individuals account for more than 80% of onward disease transmissions.

Individual-specific control

A disease intervention strategy that specifically targets potential super-spreaders, increasing individual variation in transmission and elevating the likelihood of stochastic extinction to prevent major outbreaks.