I've read that lymphoma can occur dispite of high cd4 count. Why does this happen? What should one do to prevent or cure from lymphoma?

You don't have to be HIV+ to get lymphoma, but the risk of developing it are higher for individuals with impaired immune systems, such as those with HIV or a solid organ transplant. Because even people with apparently normal immune systems can get lymphoma, there is no association with a specific CD4 cell count (unless you are talking about the more unusual kind confined to the brain and central nervous system, which occurs in people with fewer than 100 cells). There is nothing that can be done speficially to prevent lymphoma. Standard treatment consists of combination chemotherapy, which will be curative in some people.