The acts that nullify / spoil wudhu’ are four:

• when any impurity proceeds from one’s body.
   
• when consciousness is lost though insanity, sleep, drowsiness, etc.
   
• when one touches with the palm of the hand the private parts of any
  human being including one’s own (both palm and the part bare).
   
• when one’s bare body touches the bare body of a person opposite sex,
  if that person is within the marriageable degree of relationship, even if dead
  (emergency permissible).
   
• When one’s wudhu’ is nullified, one has to take wudhu’ again before
  performing the salaah.
   

********************************