Multiple infection is naturally common, as humans and animals tend to be infected by different strains of the same parasite species. This work focuses on modelling the population dynamics of co-infection which is one of the theoretical frameworks of multiple infection, based on the model devised by van Baalen & Sabelis. Furthermore, a trade-off between transmission and virulence is additionally considered due to its importance to parasite evolution theories. Then different scenarios in equilibrium are analysed by studying the effects of model parameters. Finally, several extensions are made by digging deeper into two of the parameters and relaxing one of the model assumptions.