He takes anti-allergy medication.
This was specifically referenced in a (deleted) scene for the episode "Ripple Effect"
INT. INTERROGATION ROOM #2 — DAY
Green Mitchell is seated across the table from Desert Camo Daniel. A
glass of water and a pill dispenser sit in front of Desert Camo
DESERT CAMO DANIEL: So how do we get back to our reality?
GREEN MITCHELL: We don’t know yet, but we’re working on it. In the
meantime, I’d like to ask you a few questions.
Desert Camo Daniel nods. Green Mitchell turns on the tape recorder
sitting on the table. Desert Camo Daniel opens up the pill dispenser,
pops a couple, and washes them down. Off Green Mitchell’s look –
DESERT CAMO DANIEL: Antihistamines. I have terrible allergies.
Green Mitchell nods, moves on –
and as Austin has pointed out in his comment above, this was also confirmed within "The Broca Divide"
FRAISER: Because only some of them have this low histamine level which means there is probably a-a naturally occurring antihistamine in
their diets. The ones who don't eat it eventually contract the
organism. Which is what gave me an idea. I checked the files; both
Dr. Jackson and I have acute rhinitis caused by severe allergies.
[O'Neill looks at Teal'c.]
FRAISER: I take strong antihistamines every day, so did Dr. Jackson. I have no histamine for the microbe to feed on. Just like
the Untouched, the organism starves in my body.