Hi!
You understood it correctly! Just as stated om page 337 - diseases and memes work jus as poisons with the same strengths (as described on page 113). As stated memes attack the mind. I would use WITS or EMPATHY depending on the effect of the meme. Diseases would mostly affect STRENGTH or AGILITY, but could affect WITS (neurological disease) or EMPATHY (psychiatric disease).
I hope that clarifies this?
/Kosta