  From fake news to innovative technologies, many contagions spread via a
process of social reinforcement, where multiple exposures are distinct from
prolonged exposure to a single source. Contrarily, biological agents such as
Ebola or measles are typically thought to spread as simple contagions. Here, we
demonstrate that interacting simple contagions are indistinguishable from
complex contagions. In the social context, our results highlight the challenge
of identifying and quantifying mechanisms, such as social reinforcement, in a
world where an innumerable amount of ideas, memes and behaviors interact. In
the biological context, this parallel allows the use of complex contagions to
effectively quantify the non-trivial interactions of infectious diseases.
