The area was badly affected by the outbreak of foot-and-mouth disease across the United Kingdom in 2001. The outbreak started in Surrey in February, but had spread to Cumbria by end of March. Thousands of sheep, including the native Herdwicks which graze on the fellsides across the district, were destroyed. In replacing the sheep, one problem to overcome was that many of the lost sheep were "heafed", that is, they knew their part of the unfenced fell and did not stray, with this knowledge being passed between generations. With all the sheep lost at once, this knowledge has to be relearnt and some of the fells have had discreet electric fences strung across them for a period of five years, to allow the sheep to "re-heaf". At the time of the outbreak, worries existed about the future of certain species of sheep such as Ryeland and Herdwick in the district, however these fears have been allayed and sheep now occupy the district in abundance.