Tumah Of The Cow

The parah adumah causes tumah to those that are involved in the processes of its purifying process but not to people that merely touch it.  There are two ways to understand this law.  One way we can understand that the parah is a cheftzah of tumah but the Torah limits that tumah only to those that are involved in the processes of the cow.  Another possibility is that there is no inherent tumah on the cow itself, the tumah is a din on those that are working with the cow. 

The Gemorah has a principle it discusses in a few places, notably regarding the parah adumah of סופו ליטמא טומאה חמורה.  This means that something which will ultimately in the future assume a higher status of tumah doesn’t require hechsher in order for it to cause tumah to other foodstuff.  This rule applies even before it reaches the higher level of tumah, it will immediately have the power to affect foodstuff.  The example the Gemorah gives is a bird which is dead which will cause a higher level of tumah to a person that eats it when it’s in the person’s throat.  The Gemorah says because of this rule it doesn’t require hecsher.  The Rishonim debate if it requires something tamah to make it tamah or its tamah automatically.  The Rambam understands the Gemorah in Zevachim (ibid.) to differentiate in this point between the parah adumah and the bird.  The parah adumah only will eminate tumah if its touched by something which is tamah but in regard to the bird there is no such requirement.  In his words in Shar Avos Hatumah (3:2-3) אין נבלת העוף הטהור צריכה מחשבה לטמא טומאה זו החמורה אלא כיון שבלע ממנה כזית מכל מקום הרי זו מטמאה בבית הבליעה חישב עליה לאכילה הרי זו מטמאה טומאת אוכלין והרי היא כאוכל ראשון לטומאה אע"פ שלא נגע בה טומאה אחרת ואינה צריכה הכשר:

פרה אדומה ושעירין הנשרפים אינן כן אף על פי שהן מטמאין המתעסק בהן אם חישב עליהן לאכילה צריכין שתגע בהן טומאה ואח"כ יטמאו טומאת אוכלין.  

Why is there this distinction?  In light of the previous chakirah we understand the bird is itself something which will become tamah but the parah adumah itself isn’t tamah, only the people involved in its process become tamah and therefore it needs something tamah to touch it to conduct tumah (Kesef Mishne, Mishnas Rav Aharon Taharos #27.)