Olympians returning directly from Rio will face a four-week blood-donation stand-down because of the city's being in the zone of mosquitoes that carry viruses. The stand-down period may be even longer if the athletes went to rural areas. The mosquitoes carry dengue, Zika and chikungunya viruses. Female athletes will be deferred for four weeks following sex with a man diagnosed with or suspected of having Zika within the preceding three months. Eligibility to donate blood is affected by many factors, including illness, pregnancy, breastfeeding, age, travel and potential exposure to blood-borne infections. One of the biggest obstacles facing Kiwis and immigrants to New Zealand is because of "mad cow disease". People are banned from being a New Zealand blood donor if they lived in the UK, Ireland or France for a cumulative six months or longer between 1980 and 1996. That policy was introduced in 2000. Mad cow disease, when it infects humans, is called variant Creutzfeldt-Jakob disease (v-CJD). This is a fatal, brain-wasting condition caused by a faulty protein called a prion. Those who lived in any of the three countries from 1980 to 1996 are at increased risk of developing v-CJD because of an increased likelihood they ate food from cattle infected with the cattle version, called bovine spongiform encephalopathy (BSE). The prion may be present in some people in the UK who have not developed symptoms of the disease. There is no blood-screening test for v-CJD and several cases have occurred following blood transfusion. Around 230 cases of v-CJD have been reported worldwide. Most were diagnosed in the UK and 27 in France. When the NZ Blood Service introduced its initial ban in 2000, it estimated that around 12,000 - 10 per cent - of its active blood donors were excluded, but the service does not know how many people currently living in New Zealand are ineligible to become blood donors. Blood Service medical director Dr Peter Flanagan said v-CJD was being widely researched in the UK but nothing yet indicated the donor exclusions could be relaxed.