HIV is detected in the blood after development of antibodies called HIV antibodies. Time taken for these antibodies to develop after getting HIV infection is called window period. Usually, it takes 6-12 weeks to develop HIV antibodies. A test during this window period can give false negative results because test detect HIV antibodies not virus. During the widow period, HIV is present in the blood but HIV antibodies are not present. So, this is reason that HIV go undetected for few weeks after transmission.