Correct, it's not. But it is roughly linear in the limit of small numbers of people with a small constant probability of becoming infected per interaction. (This assumption becomes problematic when you see "clustering" of highly social people with other highly social people.)
To be specific, if P is the probability of becoming infected when interacting with a single person, then the probability of becoming infected after interacting with N people is 1-(1-P)^N = NP - O(N^2 P^2). It's easy to see that the limiting infection probability is 1 in this simplified model, and that if N*P < 1 you're looking at close-to-linear growth.
Correct, it's not. But it is roughly linear in the limit of small numbers of people with a small constant probability of becoming infected per interaction. (This assumption becomes problematic when you see "clustering" of highly social people with other highly social people.)
To be specific, if P is the probability of becoming infected when interacting with a single person, then the probability of becoming infected after interacting with N people is 1-(1-P)^N = NP - O(N^2 P^2). It's easy to see that the limiting infection probability is 1 in this simplified model, and that if N*P < 1 you're looking at close-to-linear growth.