You cannot.
Contagion has a rather long cool down. You get it about once every other or every third cycle of DoT maintenance. Picking when to extend your DoTs is crucial in some fights (Leviathan Extreme comes to mind).
My rotation for SMN in Turn 8:
Aetherflow/Obey > Raging Strikes > X-Potion of Int > Miasma > Bio II > Bio > Fester/Contagion (both of these are during cool down) > Spur/Rouse > Swiftcast > Shadowflare > Fester (during cool down) > Enkindle > Ruin Spam > Fester.
Rinse and repeat as able and time everything with Battle Voice/Foe's Requiem.
SMN is more involved than BLM, generally. But the payoff is better.
