Why is HIV invisible?

Answer 1

For a doctor to find HIV is not all that difficult once he knows to look for it, is involves little more than a (granted fairly complex) blood test. However the reason our bodies are not good at detecting it is because the Virus does not act like any other. Most firuses consist of a Protein Coat and an RNA strand. The virus lands the protein coat injects the RNA into the cells where the RNA starts reproducing more viruses. The protein coat is thus left on the outside of the cell where antibodies will notice it and signall the cell to be destroyed (as usual i have summarised greatly) removing the threat of the virus. The Human Immune Virus is very clever in that it takes its protein coat into the cell with the RNA and so cannot be detected by the antigens meaning the virus is free to replicates as it wishes.

Answer 2

It's not "invisible", it's just too small to see. Viruses tend to be large as molecules go, but that's still awfully tiny.

In fact, it's too small to be seen with visible light even with a microscope. So I guess in that sense it is "invisible".