Many people do not develop any symptoms when they first become infected with HIV. Some people, however, get a flu-like illness within three to six weeks after exposure to the virus. This illness, called Acute HIV Syndrome, may include fever, headache, tiredness, nausea, diarrhea and enlarged lymph nodes (organs of the immune system that can be felt in the neck, armpits and groin). These symptoms usually disappear within a week to a month and are often mistaken for another viral infection.

During this period, the quantity of the virus in the body will be high and it spreads to different parts, particularly the lymphoid tissue. At this stage, the infected person is more likely to pass on the infection to others. The viral quantity then drops as the body's immune system launches an orchestrated fight.

More persistent or severe symptoms may not surface for several years, even a decade or more, after HIV first enters the body in adults, or within two years in children born with the virus. This period of "asymptomatic" infection varies from individual to individual. Some people may begin to have symptoms as soon as a few months, while others may be symptom-free for more than 10 years. However, during the "asymptomatic" period, the virus will be actively multiplying, infecting, and killing cells of the immune system.

Once HIV enters the human body through any of the routes mentioned in section (3), it attaches itself to a White Blood Cell (WBC) called CD4. Also, called T4 cells, they are the main disease fighters of the body. Whenever there is an infection, CD4 cells lead the infection-fighting army of the body to protect it from falling sick. Damage of these cells, hence can affect a person's disease-fighting capability and general health.

After making a foothold on the CD4 cell, the virus injects its RNA into the cell. The RNA then gets attached to the DNA of the host cell and thus becomes part of the cell's genetic material. It is a virtual takeover of the cell. Using the cell's division mechanism, the virus now replicates and churns out hundreds of thousands of its own copies. These cells then enter the blood stream, get attached to other CD4 cells and continue replicating. As a result, the number of the virus in the blood rises and that of the CD4 cells declines.

Because of this process, immediately after infection, the viral load of an infected individual will be very high and the number of CD4, low. But, after a while, the body's immune system responds vigorously by producing more and more CD4 cells to fight the virus. Much of the virus gets removed from the blood. To fight the fast-replicating virus, as many as a billion CD4 cells are produced every day, but the number of the virus too rises on similar scale. The battle between the virus and the CD4 cells continues even as the infected person remains symptom-free.

But after a few years, which can last up to a decade or even more, when the number of the virus in the body rises to very high levels, the body's immune mechanism finds it difficult to carry on with the battle. The balance shifts in favour of the virus and the person becomes more susceptible to various infections. These infections are called Opportunistic Infections because they swarm the body using the opportunity of its low immunity. At this stage, the number of CD4 cells per millilitre of blood (called CD4 Count), which ranges between 500 and 1,500 in a healthy individual, falls below 200. The Viral Load, the quantity of the virus in the blood, will be very high at this stage.

Opportunistic infections are caused by bacteria, virus, fungus and parasites. Some of the common opportunistic infections that affect HIV positive persons are: Mycobacterium avium complex (MAC), Tuberculosis (TB), Salmonellosis, Bacillary Angiomatosis (all caused by bacteria); Cytomegalovirus (CMV), Viral hepatitis, Herpes, Human papillomavirus (HPV), Progressive multifocal leukoencephalopathy (PML) (caused by virus); Candidiasis, Cryptococcal meningitis (caused by fungus) and Pneumocystis Carinii pneumonia (PCP). Toxoplasmosis. Cryptosporidiosis (caused by parasites). HIV positive persons are also prone to cancers like Kaposi's sarcoma and lymphoma.

The Center for Disease Control (CDC), Atlanta has listed a series of diseases as AIDS-defining. When these diseases appear, it is a sign that the infected individual has entered the later stage of HIV infection and has started developing AIDS. The progression of HIV positive persons into the AIDS stage is highly individual. Some people can reach the AIDS stage in about five years, while some remain diseases free for more than a decade. Measurement of the viral load and the CD4 count helps a doctor in assessing an infected person's health condition.

• Lack of energy
• Weight loss
• Frequent fevers and sweats
• A thick, whitish coating of the tongue or mouth (thrush) that is caused by a yeast infection and sometimes accompanied by a sore throat
• Severe or recurring vaginal yeast infections
• Chronic pelvic inflammatory disease or severe and frequent infections like herpes zoster
• Periods of extreme and unexplained fatigue that may be combined with headaches, lightheadedness, and/or dizziness
• Rapid loss of more than 10 pounds of weight that is not due to increased physical exercise or dieting
• Bruising more easily than normal
• Long-lasting bouts of diarrhoea
• Swelling or hardening of glands located in the throat, armpit, or groin
• Periods of continued, deep, dry coughing
• Increasing shortness of breath
• The appearance of discoloured or purplish growths on the skin or inside the mouth
• Unexplained bleeding from growths on the skin, from mucous membranes, or from any opening in the body
• Recurring or unusual skin rashes
• Severe numbness or pain in the hands or feet, the loss of muscle control and reflex, paralysis or loss of muscular strength
• An altered state of consciousness, personality change, or mental deterioration
• Children may grow slowly or fall sick frequently. HIV positive persons are also found to be more vulnerable to some cancers.