Where does transmission collide?

The passing of a pathogen from an infected host to a susceptible one, whether by direct contact, through the air, or by way of a vector such as a mosquito. Each route describes a different physical path by which the agent moves between bodies. How readily a pathogen spreads early in an outbreak is summarised by the basic reproduction number, the average count of new infections one case produces in a fully susceptible population. Delamater, Street, Leslie, Yang, and Jacobsen's 2019 Complexity of the basic reproduction number (R0) warned that this single number blends the biology of the pathogen with the contact patterns of the population, so it is not a fixed property of the microbe. The referent is a biological agent moving between living hosts, and it has nothing in common with a gear turning a shaft or a bit moving down a wire beyond the shared word.[1]

Unit

New infections per case in a susceptible population, the basic reproduction number

Recovery means

Recovered or immune hosts leave the susceptible pool, which lowers how many new infections each remaining case can produce

Measured by

Estimated from contact tracing, attack rates, and models fitted to case counts and the serial interval between infections

The passing of alleles from parents to their offspring across generations, the subject of transmission genetics and the reason Mendel's laws are stated as rules about what a parent hands on. A heterozygous parent is expected to pass either of its two alleles to a given child with equal, one-half probability. The transmission/disequilibrium test uses exactly this expectation: it counts, among affected children, how often a particular allele is transmitted rather than withheld, and a departure from one-half is evidence that the allele sits near a disease locus. Ewens and Spielman's 2003 The transmission/disequilibrium test set out how this within-family comparison detects linkage while guarding against the false signals that population structure can create. Here the thing passed along is a stretch of DNA moving down a family tree, not a germ moving between strangers or a signal moving through a cable.[2]

Unit

The fraction of alleles a heterozygous parent transmits to offspring, against the Mendelian one-half

Recovery means

Not applicable

Measured by

Counted as transmitted versus untransmitted alleles from heterozygous parents, compared with the Mendelian one-half by the transmission/disequilibrium test

The sending of information from a source to a receiver over a channel, where the message is encoded as a signal and read back at the far end despite noise. The rate at which it can be sent reliably is bounded by the channel capacity, the ceiling above which errors cannot be driven to zero however clever the coding. Shannon's 1948 A mathematical theory of communication proved that this capacity is set by the bandwidth of the channel and its signal-to-noise ratio, and that any rate below it can be made as reliable as one likes. The thing passed along is an abstract message, counted in bits per second, not a pathogen between hosts, an allele between generations, or torque through a set of gears.[3]

Unit

The reliable rate of information over a channel, in bits per second, up to the channel capacity

Recovery means

Error-correcting codes let the receiver reconstruct the intended message after channel noise, provided the rate stays below capacity

Measured by

Quantified as a bit rate and compared against the channel capacity fixed by the signal bandwidth and the signal-to-noise ratio

The passing of learned culture, such as beliefs, skills, and practices, from one person to another and from one generation to the next. Unlike genes, a cultural trait can travel vertically from parent to child, horizontally between peers, or obliquely from an unrelated elder, so its spread need not follow a family tree at all. Cavalli-Sforza and Feldman's 1981 Cultural transmission and evolution: A quantitative approach built models that treat culture as inherited information and ask how fast a practice spreads depending on which of these paths carries it. The thing passed along is a learned trait moving between minds, and it is inherited by teaching and imitation rather than through DNA, a channel, or a gearbox.[4]

Unit

The passing of a learned trait between people or generations, by vertical, horizontal, or oblique paths

Recovery means

Not applicable

Measured by

Studied by comparing traits across parents, children, and peers to estimate how strongly and by which path a practice is passed on