Cells, viruses and bacteria are organised structures of biomolecules. All of these biomolecules are constantly jiggling about due to thermal energy. Other deterministic forces, such as those driving a molecular motor from one side of the cell to another (sometimes diffusion just isn't good enough over long distances), are also at play. They are necessary to get things done and move things in a straight line!!
So whats more important for biomolecules in these systems, thermal jiggling or deterministic forces. It turns out that the thermal energy at room temperature can be calculated by the most simple of formula:
Thermal energy = KT
K is the incredible Boltzmann's constant and T is the temperature. It turns out at room temperature that
KT = 4.1 pN nm
As molecular motors work with picoNewton forces over distances of nm there will be a constant competition between these thermal forces and tiny little motors trying to move cargo from one side of the cell to the other.
It turns out I had a fish called Boltzmann (Unfortunately he died), and I have a wife called KT. What a coincidence!!