Diffusion is an important practical consideration in the design of processes involving mass transfer. From a theoretical standpoint, it is a relevant information when developing an understanding of dense-phase fluid structure. Recently, due to increase in computational power, the molecular dynamics (MD) technique has been applied extensively to the study of thermodynamic and transport properties. Diffusion is the most studied transport coefficient since it is an individual property, not a collective one like viscosity or thermal conductivity. This work investigates the role of attractive forces in the diffusion coefficient of pure simple fluids and their mixtures at infinite dilution. The intermolecular potential is broken down according to the ideas of Weeks, Chandler, and Andersen (WCA). It is then applied to study diffusion in idealized fluids (such as Lennard-Jones), idealized fluid mixtures, pure real substances and real substance mixtures. Molecular dynamics simulations were performed in the canonical ensemble with the Hoover-Nose thermostat for pure simple fluids and binary mixtures of simple fluids with one component and infinite dilution. These simulation results, that provide new data for pure fluids in the region of high temperature and intermediate densities, are compared to experimental results for real substances such as ethylene, sulfur hexafluoride and pure supercritical carbon dioxide (CO2) and a CO2/phenol mixture, where phenol is at infinite dilution. In addition, the Taylor-Aris dispersion technique was employed to measure the mutual diffusion of phenol in both liquid and supercritical CO2. Agreement between simulation and experimental data corroborates the idea that diffusion in pure and binary simple fluid mixtures at moderate densities and high temperatures is primarily determined by repulsive forces.