It's true that a significant part of the human genome is made up of endogenous retrovirus sequences. Retroviruses are a special type of virus that can integrate into the genome of its host (HIV is an example). In the ancient past, human ancestors were apparently exposed to a lot of retroviruses and probably many died as a result. Sometimes leftover fragments of retroviral genes got stuck in our genome, and as a result began to evolve cooperatively with our genes (or competitively, in the sense of transposons duplicating themselves).

In HIV-negative humans, there are no functional retroviruses left in our genome, so these aren't considered full viruses anymore, just leftover pieces. Some of these endogenous retroviral genes have since evolved important functions that benefit us, and viral infections were likely an important driver in generating novel, functional genes in our evolutionary history.