  Pathogens can spread epidemically through populations. Beneficial contagions,
such as viruses that enhance host survival or technological innovations that
improve quality of life, also have the potential to spread epidemically. How do
the dynamics of beneficial biological and social epidemics differ from those of
detrimental epidemics? We investigate this question using three theoretical
approaches. First, in the context of population genetics, we show that a
horizontally-transmissible element that increases fitness, such as viral DNA,
spreads superexponentially through a population, more quickly than a beneficial
mutation. Second, in the context of behavioral epidemiology, we show that
infections that cause increased connectivity lead to superexponential fixation
in the population. Third, in the context of dynamic social networks, we find
that preferences for increased global infection accelerate spread and produce
superexponential fixation, but preferences for local assortativity halt
epidemics by disconnecting the infected from the susceptible. We conclude that
the dynamics of beneficial biological and social epidemics are characterized by
the rapid spread of beneficial elements, which is facilitated in biological
systems by horizontal transmission and in social systems by active spreading
behavior of infected individuals.
