At least two private schools in KwaZulu-Natal have warned parents to be vigilant after several pupils tested positive for the H1N1 flu virus. Clifton School in Morningside, Durban, told parents this week that three Grade 3 pupils had been diagnosed with the virus."Should your son exhibit flu symptoms we suggest you err on the side of caution and consult a doctor," the school urged parents."Boys should not return to school until they have medical clearance to do so. Similarly, we ask that you exercise caution and keep your son at home until this medical clearance has been received."Three preschool children at Ashton International College in Ballito, on the KwaZulu-Natal north coast, were diagnosed with the virus last month.Flick Holmes, head of the junior college at Ashton, said: "We had three confirmed cases. But we had a number of children who were really unwell. There is a lot of flu going around. Because it was quite an aggressive strain of flu it appeared to be worse than it was."The children who had confirmed flu stayed away for the prescribed time and returned when the doctor allowed them to," he said.Strains of the H1N1 virus cause a small fraction of all influenza-like illnesses and a small fraction of all seasonal influenzas.H1N1 strains caused a small percentage of all human flu infections between 2004 and 2005.In June 2009, the World Health Organisation declared the outbreak of flu caused by the new strain of H1N1 as a pandemic.KwaZulu-Natal health department said the flu season had now begun."HIN1, or swine flu, is now part of the circulating seasonal influenza viruses," the department said."The vaccine on offer for this influenza season includes cover for the H1N1 strain."..