Transmission of HIV-infection generally occurs by (a) sexual contact with infected person, (b) by transfusion of contaminated blood and blood products, (c) by sharing infected needles as in the case of intravenous drug abusers and (d) from infected mother to her child through placenta. So, people who are at high risk of getting this infection includes- individuals who have multiple sexual partners, drug addicts who take drugs intravenously, individuals who require repeated blood transfusions and children born to an HIV infected mother. It is important to note that HIV I AIDS is not spread by mere touch or physical contact ; it spreads only through body fluids. It is, hence, imperative, for the physical and psychological well-being, that the HIV /AIDS infected persons are not isolated from family and society. There is always a time-lag between the infection and apearance of AIDS symptoms. This period may vary from a few months to many years (usually 5-10 years).

After getting into the body of the person, the virus enters into macrophages where RNA genome of the virus replicates to form viral DNA with the help of the enzyme reverse transcriptase. This viral DNA gets incorporated into host cell's DNA and directs the infected cells to produce virus particles. The macrophages continue to produce virus and in this way acts like a HIV factory. Simultaneously, HIV enters into helper T- lymphocytes (T-H), replicates and produce progeny viruses. The progeny viruses reeased in the blood attack other helper T- lymphocytes. This is repeated leading to a progressive decrease in the number of helper T- lymphocytes in the body of the infected person. During this period, the person suffers from bouts of fever, diarrhoea and weight loss. Due to decrease in the number of helper T lymphocytes, the person starts suffering from infections that could have been otherwise overcome such as those due to bacteria especially Mycobacterium, viruses, fungi and even parasites like Toxoplasma. The patient becomes so immuno-deficient that he/she is unable to protect himself/herself against these infections.

- Retro virus (Lenti virus family)
- Core has 2 identicle molecules of SSRNAs, enzymes (reverse transcriptase, protease). Core also has
a. Inner protein coat (P-24)
b. Outer protein coat (P-18)
- Core is covered by lipoproteins (GP-120 and GP-41)
- GP-120 has complementry sequence to CD-4 receptors present on Macrophages (HIV factory), helper T-cells etc.

# Asymptomatic phase 5-10 yrs, There is no antibody, protection in 1st (2-12 weeks) so infectivity of patients or activeness of virus is maximum is this period. This period is called Window Period (No specific symptom appear in this phase so ELISA test is negative in window period).
#AIDS related complex (ARC) :
- mild form of HIV
- swollen lymph nodes, bouts of fever, repeated episodes of diarrhoea weight loss prolonged cough.

# Full blown AIDS
- Patient become fully immune deficient in this period T- lymphocytes orCD 4 count < 200 x 106 /litre (normal CD 4 count > 900 x 106
and now this condition is called full blown AIDS.
- Tuberculosis by Mycobacterium avium.
- Candidiasis of mouth and oesophagus by Candida albicans
- Pneumonia by fungus Pnemocystis carinii.
- Cancer of skin and lymphnodes (Kaposi' sarcoma), HIV acts as an oncovirus.
- Encephalitis by Toxoplasma gondii
- Most of infections are due to oppurtunistic infections, appear when immunity become weak.

#Investigation :
- Screeing test : (ELISA) Enzyme linked immuno sorbent assay.
- Confirmatory tests : Western blot test which detects antibodies, in patient's serum.
- Non-pecific test : PCR