Halakhah zu Zavim 2:11

It is from the roots of the commandment [that it is] since God, blessed be He, distanced us greatly from the excesses and commanded us to be holy and righteous regarding food and drink and in all other matters of man. And there is no doubt that the matter of a discharge happens to a person from his constantly leaving the straight path in his foods and drinks. And that putrid, disgusting and impure excess develops in his body, from this. And it is like they, may their memory be blessed, said (Kiddushin 2b), "It is the way of excessive drinking to lead to discharge." And the Torah informed us that a man who has this in him is called impure; and impurity is a general name for anything disgusting and vile. And in our distancing ourselves from this, we acquire the trait of righteousness and balance in all of our dispositions and in all of our abilities. And it is from this matter that he is not impure from one appearance. As that fluid has not [yet] become so strong in his body. And [so,] this small amount is not an indication of his being very habituated to leave the straight path. And since that is so, it is not fitting to render him impure with a small amount - as a person is built in a way that it is impossible for him to prevent himself from not leaving the straight line at all. But in his leaving it much, he will then be called guilty; and it is fitting that he should be impure. And even though they, may their memory be blessed said (Mishnah Zavim 2:2), "There are seven ways a zav is examined: with regard to food; drink; a load; a jump; illness; a sight; or [improper] thoughts" - meaning to say if it happened to him because of this, he is not impure - our argument is not contradicted by this. As the matter is that we need to examine if his discharge was by way of a one-time event and he should not be made impure for that; or whether it has become strong in his body from a bad habit to which he has become accustomed many times, and [so,] it is fitting to render him impure. And so did Ramban, may his memory be blessed, write (in Ramban on Leviticus 15:11) and this is his language: "And the reason for the impurity of the discharge is because of its being a heavy illness, from the contagious illnesses." To here [are his words]. And if it were from the angle of an event - meaning to say from one of the [seven] well-known ways - it would not be a contagious disease, and it would not be fitting to render him impure. And we only do this inspection with the second appearance, as there is no impurity of discharge without two appearances, as we said. As so did the true explanation come to us.