What are 2 reasons why it is difficult to develop a vaccine for HIV?

1) The virus mutates rapidly. HIV has a very high mutation rate, which means that it can quickly change its appearance to evade the immune system. This makes it difficult for scientists to develop a vaccine that can target all of the different strains of HIV.

2) The virus can hide from the immune system. HIV can infect cells of the immune system itself, making it difficult for the body to mount an effective response. The virus can also produce proteins that interfere with the immune system's ability to function.