Odo's infection was implied to come from his visit to Earth, as per "Exteme Measures". Or at least the heroes' conviction on that was never contradicted.
Bashir: We believe that he became infected three years ago when he underwent medical examination at SF Headquarters.
O'Brien: "Evidently, Section 31 hoped that Odo would transmit the disease to the other Founders when he linked with them."
The stardate for that examination is in the middle of the 49XXX stardate year (aka Season 4, and approx. three years before "Extreme Measures"). That year would be concluded by Odo infecting the Link in "Broken Link". But Odo would then face judgement from the Link and be turned into a solid - effectively curing him of the disease, so that he only gets reinfected when Linking with the Female Founder during the Dominion occupation of DS9. After all, those from the Link get the symptoms much sooner than Odo does, suggesting Odo's disease was put on hold somehow.
(The intensity of shapeshifting activity of a victim may have to do with the rate of progress of the disease, though - and Odo is a young child who supposedly doesn't do much shifting by Founder standards, either quantitatively or qualitatively. So the temporary curing hypothesis may be contested.)
As for "Paradise Lost" and "Change of Heart", it should be natural to assume that anything stated about Founders in Alpha by those enemy operatives is completely false. That is, likely numbers for non-Odo Founders on Earth during the former episode are "one" (the fake O'Brien/Leyton) and "fifty-plus", with "four" virtually ruled out; and there's a good possibility that no Founders at all remain in Alpha during the latter episode.
OTOH, for all we know, there were no founders on Earth during "Paradise Lost": the one imitating Leyton had fled, and the man looking like O'Brien was one of the real Leyton's men spreading disinformation and fear when there was a dearth of true Founder activity that Leyton could exploit.
Timo Saloniemi
