Tuma is normally incurred under two circumstances.
1. Tuma resulting from an intrinsic state or condition; Inherent Tuma- טומאה עצמית.
a. Being a Metzora
b. Being a Ba'al Keri (SEE END NOTE)
2. Tuma transferred by interaction with an object that is intrinsically tamei; Transferred Tuma- טומאה מועברת.
a. Touching a Metzora or a Sheretz
b. Sitting on the bed of a Nida or Zava
c. Carrying a neveila
There is a third category which at first glance seems impossible, but after a moment's thought turns out to be surprisingly common.
3. Interaction with certain objects which are not tamei at all, which I call טומאה ע"י גרמא.
Where do we find such a thing? Where do we find that interaction with an object that is not tamei at all causes tuma in a person or thing?
We find it in the Yalkut here on passuk 19:11.
הַנֹּגֵעַ בְּמֵת לְכָל נֶפֶשׁ אָדָם וְטָמֵא שִׁבְעַת יָמִים. One who touches any dead human body will be tamei seven days.