Transmission is movement of an infectious agent between hosts. Routes include respiratory particles, direct contact, contaminated food or water, and animal vectors. A vector transfers an agent between hosts; a reservoir maintains an agent in the environment or host population. Host movement, immunity, infectious duration and contact patterns can affect spread.
An outbreak count must be compared with the relevant population. Attack rate in a stated group equals new cases divided by the number at risk, multiplied by 100. Comparing counts without denominators can mislead. Transmissibility describes how readily an agent spreads, while virulence concerns the severity of disease it causes. A highly transmissible agent is not necessarily the most virulent.
Example 1
After a shared meal, 18 of 60 people develop the defined disease. Calculate attack rate.
6018×100=30%. The denominator is the group at risk.
Example 2
Food A has 12 cases among 40 eaters; food B has 15 among 100 eaters. Which has the larger attack rate?
A:30%; B:15%. Food A has the higher rate despite having fewer cases.
Watch out: Risk comparisons need the relevant denominators; a larger group can have more cases but a lower proportion affected.