At birth, the prevalence of infection is zero or, in other words, the risk (or "chance" in daily language) of having escaped infection equals 1. Assuming a risk of infection of 10% during the first year of life, the risk of having escaped infection at age 1 year equals 1 * (1 - 0.1) or 0.9. If the risk of infection remains constant at 10%, then the risk of having escaped infection by the end of the second year equals 1 * (1 - 0.1) * (1 - 0.1) = 0.81 or 81%, etc.
