I am doing a project for my fluids class pertaining to particle tracking. As a sample application, I would like to simulate a virus traveling through a filter or (most simply) a very narrow tube in laminar flow conditions. I need the density of an individual virus particle (a virion as Wikipedia says) to resolve the buoyant forces. Right now I have found very low density viruses to be about 1.03 g/ml or about 1030 kg/m^3 (just under the density of water). I have no physical intuition, but that number seems very large for a virus. I would think if their density was almost as high as water that there would be no such thing as an "airborne virus" simply because it would sink to the ground. Does anyone have a good reference?