Outside of a human cell, HIV exists as roughly spherical particles (sometimes called virions). The surface of each particle is studded with lots of little spikes. An HIV particle is around 100-150 billionths of a meter in diameter. That's about the same as: Microns,4 millionths of an inch, one twentieth of the length of an E. coli bacterium or One seventieth of the diameter of a human CD4+ white blood cell.
Unlike most bacteria, HIV particles are much too small to be seen through any ordinary microscope. However they can be seen clearly with an electron microscope. HIV particles surround themselves with a coat of fatty material known as the viral envelope (or membrane). Projecting from this are around 72 little spikes, which are formed from the proteins gp120 and gp41. Just below the viral envelope is a layer called the matrix, which is made from the protein p17.
The viral core (or capsids) is usually bullet-shaped and is made from the protein p24. Inside the core are three enzymes required for HIV replication called reverse transcriptase, integrase and protease. Also held within the core is HIV's genetic material, which consists of two identical strands of RNA.
HIV belongs to a special class of viruses called retroviruses. Within this class, HIV is placed in the subgroup of lent viruses. Other lent viruses include SIV, FIV, Visna and CAEV, which cause diseases in monkeys, cats, sheep and goats. Almost all organisms, including most viruses, store their genetic material on long strands of DNA. Retroviruses are the exception because their genes are composed of RNA (Ribonucleic Acid).
RNA has a very similar structure to DNA. However, small differences between the two molecules mean that HIV's replication process is a bit more complicated than that of most other viruses. HIV has just nine genes (compared to more than 500 genes in a bacterium, and around 20,000-25,000 in a human). Three of the HIV genes, called gag, pol and env, contain information needed to make structural proteins for new virus particles. The other six genes, known as tat, rev, nef, vif, vpr and vpu, code for proteins that control the ability of HIV to infect a cell, produce new copies of virus, or cause disease.
At either end of each strand of RNA is a sequence called the long terminal repeat, which helps to control HIV replication.
HIV or Aids virus can only replicate itself (make new copies of itself) inside human cells. The process typically begins when a virus particle bumps into a cell that carries on its surface a special protein called CD4. The spikes on the surface of the virus particle stick to the CD4 and allow the viral envelope to fuse with the cell membrane. The contents of the HIV particle are then released into the cell, leaving the envelope behind.
Once inside the cell, the HIV enzyme reverse transcriptase converts the viral RNA into DNA, which is compatible with human genetic material. This DNA is transported to the cell's nucleus, where it is spliced into the human DNA by the HIV enzyme integrase. Once integrated, the HIV DNA is known as provirus.
HIV provirus may lie dormant within a cell for a long time. But when the cell becomes activated, it treats HIV genes in much the same way as human genes. First it converts them into messenger RNA (using human enzymes). Then the messenger RNA is transported outside the nucleus, and is used as a blueprint for producing new HIV proteins and enzymes.
Among the strands of messenger RNA produced by the cell are complete copies of HIV genetic material. These gather together with newly made HIV proteins and enzymes to form new viral particles. The HIV particles are then released or 'bud' from the cell. The enzyme protease plays a vital role at this stage of the HIV life cycle by chopping up long strands of protein into smaller pieces, which are used to construct mature viral cores.
The newly matured HIV particles are ready to infect another cell and begin the replication process all over again. In this way the virus quickly spreads through the human body. And once a person is infected, they can pass HIV on to others in their bodily fluids.
There are two types of HIV: HIV-1 and HIV-2. Both types are transmitted by sexual contact, through blood, and from mother to child, and they appear to cause clinically indistinguishable AIDS. However, it seems that HIV-2 is less easily transmitted, and the period between initial infection and illness is longer in the case of HIV-2.
Worldwide, the predominant virus is HIV-1, and generally when people refer to HIV without specifying the type of virus they will be referring to HIV-1. The relatively uncommon HIV-2 type is concentrated in West Africa and is rarely found elsewhere.
The strains of HIV-1 can be classified into four groups: the "major" group M, the "outlier" group O and two new groups, N and P. These four groups may represent four separate introductions of simian immunodeficiency virus into humans.
Group O- restricted to west-central Africa and
Group N- a strain discovered in 1998 in Cameroon - is extremely rare.
Group P- In 2009 a new strain closely relating to gorilla simian immunodeficiency virus was discovered in a Cameroonian woman. It was designated HIV-1 group P
Group M- More than 90 percent of HIV-1 infections belong to HIV-1 group M and, unless specified, the rest of this page will relate to HIV-1 group M only.
Within group M there are known to be at least nine genetically distinct subtypes (or clades) of HIV-1. These are subtypes
A, B, C, D, F, G, H, J and K.
Subtype C is predominantly found in India.
It has been observed that certain subtypes/CRFs are predominantly associated with specific modes of transmission. In particular, subtype B is spread mostly by homosexual contact and intravenous drug use (essentially via blood), while subtype C and CRF A/E tend to fuel heterosexual epidemics (via a mucosal route).
Whether there are biological causes for the observed differences in transmission routes remains the subject of debate. More recent studies have looked for variation between subtypes in rates of mother-to-child transmission. One of these found that such transmission is more common with subtype D than subtype A. Another reached the opposite conclusion (A worse than D), and also found that subtype C was more often transmitted that subtype D A third study concluded that subtype C is more transmissible than either D or A. Other researchers have found no association between subtype and rates of mother-to-child transmission.
The time elapsed between exposure to HIV and AIDS virus and appearance of symptoms is called window period. Some HIV/AIDS virus infected patients remain without symptoms for years together during the window period.
HIV is found in the sexual fluids of an infected person. For a man, this means the pre-come and semen fluids that come out of the penis before and during sex. For a woman, it means HIV is in the vaginal fluids which are produced by the vagina to keep it clean and to help make intercourse easier.
If a man with HIV or AIDS Virus has vaginal intercourse without a condom then HIV virus can pass into the woman's body through the lining of the vagina, cervix and womb. The risk of HIV and AIDS transmission is increased if the woman has a cut or sore inside or around her vagina; this will make it easier for the AIDS virus to enter her bloodstream. Such a cut or sore might not always be visible, and could be so small that the woman wouldn't know about it.
If a woman with HIV and AIDS Virus has sexual intercourse without a condom, HIV virus could get into the man's body through a sore patch on his penis or by getting into his urethra (the tube that runs down the penis) or the inside of his foreskin (if he has one).
Any contact with blood during sex increases the chance of infection. For example, there may be blood in the vagina if intercourse occurs during a woman's period. Some sexually transmitted diseases - such as herpes and gonorrhoea - can also raise the risk of HIV and AIDS transmission.
Receptive anal intercourse (i.e. being the "bottom") carries a higher risk of HIV and AIDS Virus transmission than receptive vaginal intercourse. The lining of the anus is more delicate than the lining of the vagina, so is more likely to be damaged during sex. Any contact with blood during sex increases the risk of infection.
Oral sex with an infected partner carries a small risk of HIV and AIDS infection. If a person gives oral sex (licking or sucking the penis) to a man with HIV virus, then infected fluid could get into their mouth. If the person has bleeding gums or tiny sores or ulcers somewhere in their mouth, there is a risk of HIV and AIDS virus entering their bloodstream. The same is true if infected sexual fluids from a woman get into the mouth of her partner.
There is also a small risk if a person with HIV and Aids virus gives oral sex when they have bleeding gums or a bleeding wound in their mouth. Saliva does not pose a risk.
HIV and AIDS infection through oral sex alone seems to be very rare, and there are things you can do to protect yourself.
Injecting drug users are a high-risk group for exposure to HIV virus. Sharing injecting equipment is a very efficient way to transmit blood-borne viruses such as HIV and AIDS Virus and Hepatitis C. Sharing needles and "works" (syringes, spoons, filters and blood-contaminated water) is thought to be three times more likely to transmit HIV than sexual intercourse. Disinfecting equipment between each use can reduce the chance of transmission, but does not eliminate it entirely.
An infected pregnant woman can pass HIV on to her unborn baby during pregnancy, labour and delivery. HIV can also be transmitted through breastfeeding. If a woman knows she is infected with HIV, there are drugs she can take to greatly reduce the chances of her child becoming infected.
Some people have been infected through a transfusion of infected blood. These days, in developed countries all the blood used for transfusions is tested for HIV. In those countries where the blood is tested, HIV infection through blood transfusions is now extremely rare. In some developing countries, testing systems are not so efficient and transmission through blood transfusions continues to occur.
Blood products, such as those used by people with hemophilia, are now heat-treated to make them safe.
Hospitals and clinics should take precautions to prevent the spread of blood-borne infections. These measures include using sterile surgical instruments, wearing gloves, and safely disposing of medical waste. In developed countries, HIV transmission in health-care settings is extremely rare. However, cases continue to occur in less-resourced areas where safety procedures are not so well implemented.
Health-care workers have on rare occasions become infected with HIV by being stuck with needles containing HIV-infected blood. A few have also become infected by HIV-infected blood getting into the bloodstream through an open cut, or splashing onto a mucous membrane (e.g. the eyes or the inside of the nose). There have been only a very few documented instances of patients acquiring HIV from an infected health-care worker.
Anything that potentially allows another person's blood to get into your bloodstream carries a risk. If the equipment has not been sterilized before having a tattoo or piercing, there could be a significant risk of exposure if the person before was HIV positive.
People with advanced HIV infection are vulnerable to infections and malignancies that are called 'opportunistic infections' because they take advantage of the opportunity offered by a weakened immune system.
A partial list of the world's most common HIV-related opportunistic infections and diseases includes:
Different conditions typically occur at different stages of HIV infection. In early HIV disease people can develop tuberculosis, malaria, bacterial pneumonia, herpes zoster, staphylococcal skin infections and septicaemia. These are diseases that people with normal immune systems can also get, but with HIV they occur at a much higher rate. It also takes longer for a person with HIV to recover than it takes for someone with a healthy immune system.
When the immune system is very weak due to advanced HIV disease or AIDS, opportunistic infections such as PCP, toxoplasmosis and cryptococcosis develop. Some infections can spread to a number of different organs, which is known as 'disseminated' or 'systemic' disease. Many of the opportunistic infections that occur at this late stage can be fatal.
Providing prevention and Support of opportunistic infections not only helps HIV-positive people to live longer, healthier lives, but can also help prevent TB and other transmissible opportunistic infections from spreading to others.
HIV-positive people can reduce their exposure to some of the germs that threaten their health. They should be especially careful around uncooked meat, domestic animals, human excrement and lake or river water. However there is no practical way to reduce exposure to the germs that cause candidiasis, MAC, bacterial pneumonia and other diseases because they are generally common in the environment.
Toxoplasmosis (toxo) is caused by a protozoan found in uncooked meat and cat faeces. This microbe infects the brain and can cause raised intracranial pressure, which leads to headaches and vomiting. Other symptoms include confusion, motor weakness and fever. In the absence of Support, disease progression results in seizures, stupour and coma. Disseminated toxo is less common, but can affect the eyes and cause pneumonia.
Definitive diagnosis of toxo requires radiographic testing (usually an MRI scan). The infection is treated with drugs such as pyrimethamine, sulfadiazine and clindamycin. Leucovorin may also be used to prevent the side-effects of pyrimethamine. Prophylaxis against toxo is through taking TMP-SMX.
Recommendations advise HIV-positive individuals to:
Tuberculosis (TB) is a bacterial infection that primarily infects the lungs. Tuberculosis is the leading HIV-associated opportunistic disease in developing countries. For people who are dually infected with HIV and TB, the risk of developing active tuberculosis is 30-50 fold higher than for people infected with TB alone. And because mycobacterium can spread through the air, the increase in active TB cases among dually infected people means:
Tuberculosis is harder to diagnose in HIV-positive people than in those who are uninfected. The diagnosis of TB is important because TB progresses faster in HIV-infected people. Also, TB in HIV-positive people is more likely to be fatal if undiagnosed or left untreated. TB occurs earlier in the course of HIV infection than many other opportunistic infections.
A proper combination of anti-TB drugs achieves both prevention and cure. Effective Support quickly makes the individual non-contagious, which prevents further spread of the TB germ. The DOTS (directly observed short course) Support strategy recommended by WHO treats TB in HIV-infected persons as effectively as it treats those without the virus. A complete cure takes 6 to 8 months and uses a combination of antibiotics. In addition to curing the individual, it also prevents further spread of the disease to others. This is why treating infectious cases of TB has important benefits for society as a whole.
Isoniazid preventive therapy is recommended as a health-preserving measure for HIV-infected persons at risk of TB, as well as for those with latent TB infection.