I would like to have the player character contract a disease under certain conditions and show the I have contracted message. However, player.AddSpellNS doesn't show the message, but instead shows the penalties you receive. Is there any way to show the message without the penalties? Thanks.

The "NS" variant suppresses the message, so just use AddSpell.

I tried that but AddSpell just gives the added. message, rather than the default contracted message. It also still shows the penalties.

Try this:

Message "You have contracted ."Message "  "Player.AddSpell 

That should give you the message you want without the unwanted one.

The message itself isn't too much of an issue. I would probably just read the sContractedDisease game setting, but the main problem is the penalty messages like Your agility is being drained. Normally you don't get those messages when contracting a disease.