KARACHI - Health experts have expressed their concern over the increasing cases of dengue fever in the metropolis, as 150 cases reported during the last three days.   They have warned the citizens to adopt precautionary measures against the possible attack by mosquitoes, causing malaria and dengue fever.  Talking to The Nation, a Bangladeshi scientist, Professor Dr Nurul Kabir, working at Dr Panjwani Centre for Molecular Medicine and Drug Research (PCMD), Karachi University, said that the government was required to control the spread of dengue fever in the country for which mosquito control was the primary means of prevention.  “As many as 50 million dengue patients are reported annually in the world, while the disease has been spread almost 100 countries. Around 250 million people are on the risk.