Hillel is the man who rescues Passover in the last decades before the Common Era. When the elders don't know how to prepare for a Saturday night Seder, it is Hillel who teaches them what to do (Tosefta, Pesahim 4:13). When others cannot figure out what to do with lamb meat, flat bread and salad, he invents the shawarma (Babylonian Talmud, Pesahim 115a).
Hillel went up from Babylonia because of three matters. The verse says, "He is pure" (Lev. 13:37). Does this mean that if the symptoms disappear, he does not need the priest? No, for the verse continues, "The priest shall declare him pure." But what if a priest said "pure" to one who was really impure, does he thereby become pure? No, for the verse says, "He is pure; the priest shall declare him pure." For this Hillel went up from Babylonia.Hillel not only provides practical Passover direction for his contemporaries, he also resolves their textual difficulties: the passover lamb or goat is for dessert (i.e. afikoman), but the main course can be beef; matzot can be made throughout the week from the old flour, but the new flour cannot be used until day two, when the Omer is offered.
One verse says (Deut. 16:2), "You shall slaughter the passover for the Lord, flock or herd," but another says (Ex. 12:5), "From the sheep or the goats you shall take them." How is this? The festival offering can come from either, but the passover can only come from the flock.
One verse says (Deut. 16:8), "You shall eat matzot for six days," and another says (Ex. 12:15), "Seven days shall you eat matzot." How is this? Six days of the new crop, seven days of the old crop.
Hillel expounded, and his conclusions were confirmed. He went up to Israel and it was accepted as law.
But one of these things is not like the others. The first matter Hillel comes to teach is about the purification... of tzaraat. This plague is catching like... like... Anyway, here we go again with the Hansen's disease.
It doesn't seem that Hillel's first exegesis is really about tzaraat; far more significantly, it demonstrates his halakhic approach. When approaching the inverse verse, "The priest shall declare him impure... he is impure," one might be tempted to say that impurity can be assigned on one of two bases: objective reality (he is impure) or subjective considerations (he has been declared impure). After all, forbidding a given act or item on halakhic grounds is temptingly easy for any decisor. Even if something is technically permissible, there are always a handful of ancillary reasons to prohibit.
However, Hillel's first lesson is the verse which disproves this approach: "He is pure; the priest shall declare him pure." When he is pure, the priest must declare him so; this is a sacred duty. Ultimately, Hillel and his followers gain a reputation of being generally lenient (unlike the generally stringent approach of his colleague Shammai), but the Mishna devotes an entire chapter (Eduyot 5) to listing the exceptions to this rule. Hillel is not lenient for the sake of being lenient; he is lenient because that is what the objective facts require. The solutions he finds for the observance of Passover reflect the fact that his first and foremost dictum is "He is pure; the priest shall declare him pure."
As we approach Passover, it's worth remembering what the Talmud says (Eruvin 6b):
The halakha is always in agreement with Beit Hillel, but he who wishes to act in agreement with the ruling of Beit Shammai may do so, and he who wishes to act according to the view of Beit Hillel may do so; he, however, who adopts the more lenient rulings of Beit Shammai and the more lenient rulings of Beit Hillel is a wicked man, while of the man who adopts the restrictions of Beit Shammai and the restrictions of Beit Hillel, Scripture says (Eccl. 2:14): "But the fool walks in darkness."