wood bunks,  dead drees,  piles of  decomposed manure all  cellulose based  items,  are great harboring places for the fungi  to  stage  and wait to be rubbed on by cattle.  
Where it comes from?   it's everywhere... and seems to get more concentrated if the same  bunks/ dead trees/manure mounds  stay in the pen  after year.  
keeping the calves  free of other parasites  helps with the rubbing, and exposure to the fungi.  
I had  a  bunch of  dead stumps I thought were good scratching posts in my weaning pens  and every year had an outbreak, even though the pens were empty 9 months out of the year.
  vet told me to get rid of the stumps,  and  problem solved- well, greatly reduced anyways.  

have had good luck using  diesel fuel and iodine  to spot treat.