The immune system is the body’s natural defence that protects it against disease and ill health.  HIV is a virus which weakens the human immune system.  HIV stands for human immunodeficiency virus.  If left undiagnosed and untreated HIV can cause damage to the immune system which can leave people vulnerable to other infections.


AIDS stands for acquired immune deficiency syndrome, and is not the same as HIV.  It is unlikely that the term AIDS would be used these days, particularly in healthcare.  Advanced or late-stage HIV is how this would be referred to.  Even if someone becomes very ill with HIV, if it is diagnosed and treated in time for the immune system to recover, then usually the person can recover to some extent.  Early diagnosis and treatment are the key to avoid this happening.

You cannot catch AIDS and there is no such thing as an AIDS test.

The word ‘AIDS’ is often misquoted by many people including the general media.


AIDS stands for Acquired immune deficiency syndrome, and is not the same as HIV. In fact, in the UK, we rarely use the term AIDS, but use the term Advanced HIV instead.


The term AIDS is correctly used to describe a situation when someone’s immune system has been severely weakened by the virus HIV. As this person’s immune system is damaged, they are more likely to pick up other infections and diseases. Common infections include PCP (a certain type of pneumonias called Pneumocystis pneumonia), Tuberculosis and certain cancers (for example the skin cancer Kaposi's Sarcoma [KS]). These infections are also known as opportunistic infections as they are rarely seen in people with healthy immune systems.


A common misconception is that AIDS can be passed on. This is not the case. HIV, the virus which can lead to AIDS (Advanced HIV), can be passed between individuals, but AIDS is merely a term used to describe a collection of illnesses.



Different kinds of sex have different risks attached, anal sex has the highest risk, followed by vaginal sex.  There is also a risk with oral sex, but the risk is greatly reduced.  HIV is present in sexual fluids - semen, pre-ejaculatory fluid, vaginal fluids, and anal mucus and this is why it can be passed on during sex.  Whether HIV is transmitted is dependent on numerous factors, and these can be quite complex.  It is known that viral load is implicated in transmission and the higher the viral load, the higher the risk of transmission.




HIV is also present in blood and there are other ways in which HIV can be transmitted.  These are through sharing injecting equipment when taking intravenous drugs, or anything else that can expose someone to someone else’s blood.




HIV can also be passed from a mother to her child through pregnancy, during birth or breast milk.  Mother-to-baby transmission in pregnancy is extremely uncommon in the UK, as doctors work with pregnant women to reduce the risk of this happening.  Less than 1% of babies born to women living with HIV in the UK are born with HIV themselves.

The CD4 count and viral load are the two measures which are most used to determine how HIV may be impacting on the immune system and how much HIV is in the body.



This measure is an indication of the strength of the immune system.


CD4 cells are an extremely important part of our immune system, HIV attacks these cells which can affect the number of them in the body.  Doctors will test a cubic ml of blood, and use this to determine the CD4 count i.e. the number of CD4 cells in the sample.  This will then allow the doctor to see how healthy a person’s immune system is. The higher the CD4 count, the healthier the immune system is.


However, it is important to acknowledge that your CD4 count will have natural variations, and these do not mean that you necessarily have a stronger or weaker immune system.


It has been said that a more reliable figure to give an indication of the immune system is the CD4 percentage.  Your HIV team will have both your CD4 count and your CD4 percentage and if you ask, they will be more than happy to discuss your results.



This measure indicates how much HIV there is per cubic millilitre of blood.

The lower this number, the less HIV is in the system and the less damage it can cause.  The aim of putting people on treatment is to get the viral load count to undetectable levels.  This means that the amount of HIV in the system is at such low levels that ill-health is unlikely due to the HIV and there is zero risk of passing on HIV. 

Viral loads will vary between individuals, but most people will quickly achieve an undetectable viral load once treatment has started.




This is mainly only achieved when taking HIV medication.  The term undetectable viral load can be misleading to some people.  Although it is called undetectable, there is still HIV present in the body.  It is called undetectable as the devices used to determine the viral load cannot detect HIV if there are fewer than 40 to 50 copies of HIV per cubic millilitre of blood.


 Having an undetectable viral load is the aim of treatments and means that:


  • Treatments you are taking are working.


  • HIV is present in very low levels.


  • HIV is causing less damage to the body.


  • There is zero chance of passing on HIV with an undetectable viral load.



If someone is not on treatment, the viral load and the CD4 count may be factors that determine when the doctors think it is appropriate to start doing so.  However, current UK guidelines state that anyone living with HIV can start treatment when they feel that the time is right.  You can talk to your HIV consultant or nurse specialist about this.