Complex sentence: Parvovirus B19 ("B19 virus") was the first (and, until 2005, only) human parvovirus to be discovered, by chance in 1975 by the Australian virologist Yvonne Cossart.
Split into simpler sentences: Parvovirus B19 ("B19 virus") was the first (and, until 2005, only) known human parvovirus. It was discovered by chance in 1975 by Australian virologist Yvonne Cossart.