Abstract: Pathogens can be transmitted both vertically (from the parent to the offspring) and horizontally. Here, I model the co-evolution of pathogens and their hosts allowing for vertical and horizontal transmission and density-dependent host population growth. My analysis uses evolutionary game theory. I use computational methods to find that increasing vertical transmission does not always result in more benign disease outcomes. Instead, it can lead to higher pathogen-induced mortality. Furthermore, more benign outcomes evolve more readily when horizontal transmission is more profitable for the pathogen, and overall virulence increases as horizontal transmission becomes more profitable. The results also indicate that vertical transmission, when associated with high virulence, can drive selection-driven extinction of the pathogen, which highlights the importance of considering both transmission modes in evolutionary studies.