Figure. Probability of attack detection delay for a contagious agent. Dashed line represents the early approximation , solid lines the full solution (where the quoted numbers represent the fraction p of the population initially infected), and the symbol “nc” stands for noncontagious agent (). The parameters are as follows: blood donation rate k = 0.05 per person per year, screening mean window period ω = 3 days, mean duration of infectiousness 1/r = 14 days, basic reproductive number , and the initial attack size Np = 500. Note that the exposed populations are therefore 5,000 and 625 for p = 0.1 and , respectively.