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Initial acute infection is quite mild and is characterized by fever (80-90% cases), fatigue (70-90%), rash (40-80%), headache and lymph node enlargement. The acute infection usually lasts for less than two weeks and subsides. Once inside the body, the virus spreads by attaching mainly to white blood cells. This is a highly infectious stage where the infected person can pass on the infection to a sex partner easily.

Antibodies against the virus do not start developing for 3-4 weeks after first being infected.1 It is therefore hard to diagnose a case during the acute infection stage.

In one study, of 23 persons who were enrolled in a routine HIV surveillance program and became HIV positive, 89% developed an acute infection. Of those who developed an acute infection, although 95% went to the doctor, only 25% were diagnosed with acute HIV infection.2

How is HIV Diagnosed?

•Diagnostic tests look for the HIV virus or parts of the virus (antigen) or for molecules against the virus in the blood (antibodies)
•Traditionally, an ELISA (enzyme linked immunoassay) test is used to detect HIV antibodies
•ELISA can show positive results after 3-4 weeks1
•A new rapid antibody test takes < 20 minutes2
There are a host of tests for detecting HIV infection. Typically, those that detect parts of the virus (like HIV RNA or plasma p24 antigen) can identify an acute infection before antibodies are produced by the body.

The traditional ELISA tests generally do not show positive results during an HIV acute infection until 3-4 weeks after the exposure; that is—after the body produces antibodies against the virus. CD4 (a certain type of white blood cell) cell count is used primarily for determining the stage of HIV infection. 
 
Early detection of HIV infection is important for preventing HIV transmission because people are most infectious during the acute infection. Also, after the initial symptoms of acute illness that disappear within a couple of weeks, the infected person may be asymptomatic and undiagnosed for many years.
 
References:
1. Spach DH. HIV and AIDS. In:  Dale DC, ed. Infectious Diseases: The clinical guide to diagnosis, treatment, and prevention. New York: WebMD. 2003:228.
2. CDC. Notice to Readers: Approval of a new rapid test for HIV antibody. MMWR [serial online]. 2002;51(46);1051-1052. Available at: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5146a5.htm. Accessed March 10, 2004.