I played around with a similar deck in the trainer before. 4 aflatoxin and the death nymph is probably overkill, 3 aflatoxin and the nymph would still have the same effect. Also, improved plague is much more reliable than the retrovirus as retrovirus is open to all kinds of CC. 3 skull bucklers is probably also overkill since you are running PA's 2 would be plenty sufficient.
Since you have the altatoxin + mass poison combo, I would look at either a graveyard, Condor, or both as a means of adding damage to this deck as it is almost a pure stall deck as of now.