DS9 "When it pours..."
BASHIR [on monitor]: I was running a scan on the sample you left when I noticed something unusual.
ODO: Unusual in what way?
BASHIR [on monitor]: The disease that's killing your people. You're infected. My guess is you became infected the last time you Linked with the Female Shape shifter.
KIRA: Are you sure about this? He doesn't have any symptoms.
BASHIR [on monitor]: So far. But from what I can tell, the virus is replicating quickly and it may not be long before symptoms start to surface.
Section 31 infected odo with a disease so that he would spread it to the founders and the part of that statement that Bashir said about the female founder infecting him was not true it was actually the opposite.
BASHIR: Hang on, hang on, I'm just trying to figure out when Odo became infected. I analysed the sample he left and mapped out the life cycle of the virus. I'm programming the computer to calculate just how long it took for the virus to achieve its present level of concentration.
O'BRIEN: That makes sense. According to that, he was infected over two years ago?
BASHIR: More than that. I just don't understand. I assumed he became infected when he Linked with the female shape-shifter.
O'BRIEN: But if it happened that long ago, how come he hasn't shown any symptoms before?
BASHIR: Good question. Got it. Stardate four nine four one nine.
O'BRIEN: Almost three years ago.
BASHIR: Wait a minute. Four nine four one nine. That's the day that Odo was at Starfleet Medical.
O'BRIEN: You sure?
BASHIR: Yes. My god. No wonder Hilliard wouldn't let me see the file. Section Thirty one must have created the virus, infected Odo, and then used him as a carrier.
O'BRIEN: So that when he Linked with another changeling, he'd pass it on, and eventually the entire race would be infected.
BASHIR: Thirty one isn't just trying to stop us from finding a cure. They're trying to cover up the fact that they set out to commit genocide
So we know Section 31 created it. We also know that it was two years in the past. Odo didn't show any signs for those two years, and after his bonding with the female changeling she became infected and showed signs fairly quickly.
She showed signs before Odo did and he was infected two years before her.
Bashir said in the episode he couldn't figure out why Odo showed signs later when he was infected so much earlier.
Do we ever learn why Odo showed signs of the disease later when he contracted it two years earlier?