Two of them can be infallible:
1) The Holy Ghost is always infallible.
2) The Magisterium is infallible when:
a) The Pope speaks ex cathedra
b) Bishops, in union with Pope, define doctrine at
a General Council
c) Bishops propose definitively, dispersed, but in
unison, and in union with Pope
See:
http://www.newadvent.org/cathen/15006b.htm