The Gemoro (Nedorim 7b) explains that the Torah's statement that the troublemakers in Mitzraim had died means that they became poor. The Ran explains that it cannot be that they got Tzora'as, because the Torah later says that they were BeKerev Kol Yisroel, and it cannot be that they were healed at Matan Torah because all such illnesses returned after the Egel.
Why could it not be that they got Tzora'as but were healed at some time before Matan Torah, so it didn't return after the Egel?
Meir Eliezer Bergman
They could only have been healed before Matan Torah if they did Teshuvah, but we know that they did not do Teshuvah because the Gemara in Megilah 11a states that Dasan and Aviram were Resha'im from their beginning to their end.
There are other reasons why it could not be that they were healed before Matan Torah. Even though Hash-m said to Moshe (Shemos 4:19) that all the men who "sought your soul" have died, this does not mean that Dasan and Aviram literally wanted to kill Moshe themselves. Rather, they wanted to inform on him to Pharaoh. We see this from Rashi to Shemos 2:15, where the Chumash states that Pharaoh heard that Moshe had killed the Mitzri. Rashi says that he heard because the two troublemakers informed on Moshe.
We also learn this from the Ran in Nedarim 7b (end of DH she'Ne'emar). He writes that it cannot be that when the Torah says that the troublemakers had died that this means that they had no children, because the fact that they had no children would not prevent them from informing to the king. We see that Moshe was afraid that they would inform on him to the king.
According to this, it could not be that Dasan and Aviram became ill with Tzara'as after Bnei Yisrael left Egypt but before Matan Torah, because as soon as they left Egypt Moshe was no longer in danger since they could no longer inform on him to Pharaoh.
In addition it could not be that they were healed of Tzara'as before Yetziyas Mitzrayim, because if that were the case, then immediately upon becoming healthy the danger they posed to Moshe would return and the promise of Hash-m that the troublemakers had "died" would now be useless.
Finally, I would just point out that even if the Tzara'as would have been healed before Matan Torah, it would still have returned after the Egel ha'Zahav as a punishnment for making the Egel.