Corrected entry: Odo would have already been infected by the virus by this point. He passes the infection to The Great Link in episode 4-26. It's highly unlikely they would have found a cure on this planet with little medical supplies and the elder Odo never mentions it. Odo should not have survived to see the crew of the Defiant return.
Correction: The virus is accelerated by shape-shifting. The Founders were affected quicker than Odo, even though he was infected first, because they change their form more often. If Odo did not change much during the 200 years, he would have remained unaffected.
Correction: At the end of Episode 4-26 Odo is turned, according to Doctor Bashir, into human and does not have any contact with an Changeling from the Great Link until the Dominion captures DS9 at the end of Season 5. Since no one was ever worried that the Changeling Virus was any danger to humanoids it's quite probable that Odo was "healed" by his people's punishment and did not carry the Virus during any Episode in Season 5, including this one.
HTH