The window period refers to the time after infection and before seroconversion, during which markers of infection (HIV-specific antigen and antibodies) are still absent or too scarce to be detectable. Standard screening tests cannot reliably detect HIV infection until after the window period has passed. Window period differs, depending on which kind of test you take.
During this window period, virus can be spread via high-risk behaviours. The window period for HIV-antibody is usually three months. Therefore, we highly recommend sexually active people to take HIV antibody test once every three months.
Nowadays, HIV antigen tests are also available. The p24 antigen test detects the presence of the p24 (a protein component of the HIV viral core) and is used for diagnosing early HIV infection. The window period is about 2 weeks. However, as p24 level drops rapidly after antibodies have been produced (about 4 weeks after the infection). The p24 antigen test is not always reliable for diagnosing HIV infection after its early stage.
HIV Test for p24 Antigen and Antibody Test (Combo Test) detects both. It reduces the window period for an HIV test to 4 weeks after exposure, and improves the accuracy of screening.
Assuming that a person has not been diagnosed (not tested) until the period of appearance of a significant symptom (AIDS defined), this period can be called latency. There is natural variation of the progression of the virus within the individual infected persons. In general, latency can be 5-10 years. As there is no symptom and no treatment, an infected person can spread the virus through high risk sexual activities.