I hope you can help me with the timeline here. But in Unworthy Thor #5, Hela, Proxima and Black Swan all met Thanos back at the Black Quadrant. It's then that Hela kisses him. But in Thanos #1, Corvus still had the Black Quadrant, and after defeating him then Thanos shows he has the god cancer.
So he must have been infected prior to the kiss, and prior to controlling the Black Quadrant. Unless the timeline in Unworthy Thor is even further back prior to Thanos #1, where he lost the Black Quadrant and came back to reclaim it, and the god cancer is running its course and getting stronger by then.