There is a physical difference between the discharge of a Zav and the discharge of a Ba'al Keri. They come from the same place in the body and look pretty much the same, but there are slight differences between them. (See also Nidah 35b and our Background to the Daf for Nidah 72:5 and 72:6 where we discussed the laws of Zav and Ba'al Keri in detail.)

In a nutshell, the laws of a Zav are as follows:

A man who sees a Zav discharge one time is Tamei for one day. If he immerses in a Mikvah during the day he becomes Tahor at night. (That is, his Halachah is the same as a man who sees Keri.)

A man who sees the Zav discharge twice must wait until seven days pass during which he does not see any Zav discharges (Shiv'a Nekiyim). After seven days, he must be Tovel in a stream or well of running water. He does not need to bring a Korban.

A man who sees the Zav discharge three times must bring a Korban, in addition to the laws recorded above that apply to a Zav that saw the discharge twice.

D. Zupnik