לע"נ ר' שמעון בן ר' חיים אליעזר
לזכות ר' אברהם שרגא בן מלכה אסתר וכל ב"ב
The carcass of a dead animal is a primary source of impurity. An olive-sized portion of its meat imparts impurity to a person and keilim when touched and to an earthenware container when it enters its inner space. And it imparts impurity to a person when being carried to the extent that his clothes become impure, as is the law regarding a surface on which one rides that contracted impurity from a zav.
What is implied? When a person touches the carcass of a dead animal, he is considered as a primary derivative of impurity - rishon li-tumah. If he touches keilim - even at the same time that he is touching the carcass - they are pure. Similarly, the clothes that he is wearing are pure, for he is a derivative of impurity and a derivative does not impart impurity to keilim.
When, by contrast, one carries the carcass of a dead animal, he imparts impurity to keilim while he is carrying it, as Leviticus 11:28 states: "One who carries their carcasses must launder his garments." Those garments are considered as primary derivatives of impurity. He does not impart impurity to another person or to earthenware containers even when he is carrying the carcass, as we explained in Hilchot Metamei Mishkav UMoshav. [Hilchos Shear Avos Hatumah 1-1]
In the gemara in Bava Kamma [2a-2b]
גבי טומאות תנן אבות הטומאות השרץ והשכבת זרע וטמא מת
With regard to ritual impurities, we learned in the mishna (Kelim 1:1): The primary categories of ritual impurity are as follows: Any of the eight species of the creeping animal enumerated in the Torah (Leviticus 11:29–30), and semen, and impurity imparted by a human corpse
תולדותיהן לאו כיוצא בהן דאילו אב מטמא אדם וכלים ואילו תולדות אוכלין ומשקין מטמא אדם וכלים לא מטמא, הכא מאי
[In this connection] their subcategories [toldos] are not equal to them in law; for a primary defilement contaminates both human beings and utensils, while subcategories defile only foods and drinks, leaving human beings and utensils undefiled. Here [regarding nezikin - damages] what is the halacha?
The Nachalas Dovid asks how the gemara can compare dinei Tumah and dinei nezikin. The reason that the toldos of tumah have a different law is because their tumah was derived from the Av and is thus less stringent while this is not the case when discussing toldos of nezikin?!!
It would appear that you cannot say that Av Hatumah applies only to the tumah itself while a tolda is what derives from the tumah of an Av, because if a person or kli touch a corpse, they become an Av Hatumah themselves even though their tumah was derived from another source. Therefore we must say that it is a גזירת הכתוב what is מטמא a person and keilim and that is called an אב even though it is not גוף הטומאה and what is not an אב is not מטמא אדם וכלים even though it is גוף הטומאה. The gemara is merely drawing from the language of the Tanna that what is called a תולדה is לאו כיוצא בהן.
In the words of the Nachalas Dovid:
"ואין להקשות דמאי מייתי מטומאה דלאו כיוצא בהן נמי תולדה מיקרי, דלמא התם משום דהיא נמשכת מהאב משום הכי מיקרי תולדה דנולדה מהאב, דטמא מת וחלל חרב יוכיחו דנמשכות ג"כ ואפ"ה אבות מיקרי".
It is also important to know that the term "אב" does not just refer to a more severe form of tumah but rather it is an essentially different level of tumah. The proof is that a ראשון and שני combine to contaminate as a שני [Taharos 1-5] and we see from Meilah 17b that the reason is that they are in the same category. But an אב and תולדה do NOT combine to contaminate because they are completely separate categories. [See Chazon Ish Taharos 1-10] So an אב is an independent category that contaminates אדם וכלים.
Also, Rashi writes in Pesachim [17a] that food that touches a corpse is an אב insofar as that it generates a ראשון but is not מטמא אדם וכלים since food cannot become an אב. This shows that even though it has the stringency of an אב insofar as making something that comes into contact with it a ראשון, it is not, strictly speaking defined as an אב.
[עפ"י הס' טעם ודעת]
