The particle measures about 0.7 micrometres (thousandths of a millimetre) in diameter.
"It is bigger than some bacteria," explained Prof Jean-Michel Claverie, from Aix-Marseille University, Marseille, France.
"You don't need an electron microscope to see it; you can see it with an ordinary light microscope," he told BBC News.
Viruses cannot copy themselves; they need to invade a host cell if they want to replicate.
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