A virologist amongst orchids

In Roman times, the term "virus" meant "poison, venom, the rammish smell of armpits, water matter, whitish, yellowish and greenish at the same time which issues out of ulcers and stinks very much, being induced with eating and malignant qualities." By 1957, the French microbiology Andre Lwoff declared, rather circularly, "Viruses should be considered as viruses, because viruses are viruses." But what really is a plant virus?