The word retroviruses is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 17
Highest Scoring Words With Friends™ Plays In The Letters
Seven Letter Word Alert: (60 words)
erosive, erotise, estrous, oestrus, ousters, outsees, overset, oversit, overuse, reissue, reroute, resorts, resters, restirs, restive, restore, retirer, retires, retours, retries, reverts, reviser, revises, revotes, revuist, rioters, riveter, roister, rosiest, rosters, routers, rustier, serious, servers, sievert, soirees, sorrier, sorters, sorties, sourest, stereos, stirrer, stivers, stories, stourie, striver, strives, suiters, suitors, terrier, terroir, terrors, toruses, tourers, trouser, ureters, verists, vetoers, virtues, viruses