The outbreak of epidemics, the rise of religious radicalization, or the motivational influence of fellow students in classrooms are some of the issues that can be described as diffusion processes in heterogeneous groups. Understanding the role that interaction patterns such as homophily, or segregation, play in the diffusion of certain traits or behaviors is a major challenge for contemporary societies. Here, we study the effects on diffusion processes of mixing (or segregating) two different groups –one group that is more sensitive or prone to “infection”, and the other which is more resistant –. We find non-monotonic effects of mixing, and Pareto inefficient segregation levels, e.g., situations where an increase in mixing can benefit both groups. These findings have fundamental consequences for the design of inclusion policies.