As Salamander said, plaguebearers should be 10 or 30, I have a thread on building a Nurgle army that explains that and other advice. Unfortunately I'm on my phone at so can't snag the link, but a quick search of 'Building a Nurgle Army' on this sub forum should find it.
For list-specific advice, first off drop the heralds. A herald of Nurgle isn't worth the 100 points charged for it so doesn't have much of a use right now. You'll need two units to replace them, and unfortunately a daemon prince won't work (otherwise it would be a great choice). I would recommend another unit of 10 plaguebearers and Epidemius
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