At the time it was just fantasy and as fantastic an idea as its title suggested. Today, however, micro-miniature travelers in your body have come one step closer to reality. Researchers from the Max Planck Institute have been experimenting with real micro-sized robots that literally swim through your bodily fluids and could be used to deliver drugs or other medical relief in a highly-targeted way.
The microrobots being designed by the team literally are swimmers; they are scallop-like devices designed to paddle through non-Newtonian fluids like blood and plasma (even water behaves in this way at a microscopic level). This means that, unlike swimming in water at a macro-level, these microbots need to move through fluid that has a changing viscosity depending on how much force is exerted upon it.
To do this, the microbots need a method of propulsion that can fit in their tiny bodies as well as take advantage of the non-Newtonian fluid in which they are moving. Importantly, the team is using a reciprocal method of movement to propel their microscallops; but generally this doesn't work in such fluids, which is why organisms that move around in a biological system use non-reciprocating devices like flagella or cilia to get about.