In Example 5, we modeled a measles pathogenesis curve by a function f. A patient infected with the measles virus who has some immunity to the virus has a pathogenesis curve that can be modeled by, for instance,(a) If the same threshold concentration of the virus is required for infectiousness to begin as in Example 5, on what day does this occur?

(b) Let P₃ be the point on the graph of t where infectiousness begins. It has been shown that infectiousness ends at a point P₄ on the graph of t where the line through P₃, P₄ has the same slope as the line through P₁, P₂ in Example 5(b). On what day does infectiousness end?
(c) Compute the level of infectiousness for this patient.

g(t) = 0.9f(1).