For the person to get infection with HIV Virus, they must have to acquire the virus from external sources, the only external source is another human being who is infected with HIV virus. That human being can be his/her sexual partner, or partner who shares injecting drug needle. People can also get infection with HIV Virus during birth via birth canal, during breast feeding or via blood transfusion/organ transplant.
But all people who are exposed to HIV Virus do not get infection with HIV. Some people are Lucky!
Wondering how luck can protect from HIV Infection? or thinking these people might have used some preventive measures such as use of condoms or screening of blood/organ, not sharing needles etc! I am not talking about those preventive measures, but about their genetic makeup which makes them resistant to HIV Virus infection.
In General, each virus follows the same sequential steps during infection process. Which is; attachment, penetration and uncoating, replication and biosynthesis, assembly and release. So the very first and crucial step is attachment of virus to their (specific) host cells. One of the good news for human is virus are very specific regarding their attachment to host cells.
Whenever we try to climb tress or very steep slope (mountains), we need something to assist us to adhere there. That may be rope or branch of tree, or rough surface. If these things are not available, can you imagaine sucessfully climbing mountain or hills or tress? Its impossible.
Similarly, for the virus to attach to host cell, they need “something (like rope or branch of tree)” which help them to make firm grip. That “something” is Receptor. Incase of HIV Virus these receptors are CXCR4 and CCR5. HIV Virus can attach only to those CD4 cells which have specific chemokine receptor CXCR4/CCR5. If that CXCR4/CCR5 receptor is lacking in particular CD4 cells, then the HIV Virus can not attach to these CD4 cells. So the whole process of HIV Virus infection/replication is halted before it can start.
Studies have found that mutations in the gene encoding CCR5 endow individual with protection from HIV infection. People who are homozygotes are completely resistant to HIV Virus infection where as among heterozygotes HIV infection progresses very slowly.
Some of these lucky HIV resistant people lives in Western Europe (Who knows, many people in your country might also be resistant to HIV Virus). Approximately, 1% of people of western european ancestry have homozygous mutations in their gene and about 10-15% are heterozygotes.