Hong Kong authorities activated a newly created serious response level today as fear spreads about a mysterious infectious disease. <br />'' <br />'' Five possible cases have been reported of viral pneumonia that may have been brought back by visitors to a mainland Chinese city. The disease has infected at least 44 people, 11 of whom were critical, in Wuhan. <br />'' <br />'' The outbreak emerged last month and has revived memories of the 2002-2003 SARS epidemic.<br />''  <br />'' The serious response level is the second-highest in a three-tier system. It is part of a new government plan launched to respond to infectious diseases of unknown cause. <br />'' <br />'' The city's health department also added a thermal imaging system at Hong Kong airport to check the body temperature of arriving passengers. <br />'' <br />'' City leader Carrie Lam, while reviewing health surveillance measures, urged travellers who develop respiratory symptoms to wear surgical masks and seek medical attention. <br />''  <br />