by Luc Montagnier et al., Interdiscip Sci Comput Life Sci, 2009
Montagnier and his colleague Lavallee observed that under certain conditions, biological fluids containing infectious microorganisms can still cause infection after being passed through filtration procedures designed to sterilise the fluid.
A fluid containing human white blood cells (lymphocytes) infected with a microorganism called Mycoplasma pirum was filtered through filters with either 100nM diameter holes or 20 nM holes; as M. pirum are 300nM in size they could not pass through and the resulting fluids (called filtrates) were apparently sterile.