The probability of transmission in epidemiology is defined as which of the following?

• A. Proportion of contacts which result in transmission
• B. Number of contacts per unit time
• C. Duration of infectiousness
• D. Incubation period

Answer: (A)

The probability of transmission is the likelihood that a single contact between an infectious person and a susceptible person results in infection. It is naturally expressed as a proportion: among all contacts that could transmit the pathogen, the fraction that actually leads to transmission is the probability per contact. For example, if 50 close contacts occur and 5 transmissions result, the probability per contact is 5/50 = 0.10 (10%). This concept is different from how many contacts happen per unit time (the contact rate), from how long someone remains infectious (duration of infectiousness), and from the incubation period (time from infection to illness). The per-contact transmission probability depends on factors like the pathogen’s infectiousness, the route of transmission, exposure dose, and host susceptibility, and it helps explain transmission dynamics alongside measures such as the secondary attack rate and overall metrics like R0 when combined with contact patterns and duration of infectiousness.