Through time people have been mobile, for a variety of complex reasons at a range of scales from local through regional and continental to intercontinental. Some are voluntary, others are made under pressure and the force of various circumstances, with environmental, socio-economic and political factors often interrelating to cause movements. We are now more mobile than at any time in the past, there are more people to move, the means to move are greater, and political boundaries are either open or often crossed without great difficulty. These movements have considerable consequences for human health, for disease transmission and diffusion and for measures for health improvement. They are viewed at a global scale, and then for two major regions in more detail to illustrate their significance for one old and one new disease, for malaria in Southeast Asia and for HIV/AIDS in West Africa.