Complex sentence: The immunodepressive action was the initial normal behavior of the virus, similar to HIV, the fusion proteins were a way to spread the infection to other cells by simply merging them with the infected one (HIV does this too).
Split into simpler sentences: The immunodepressive action was the initial normal behavior of the virus, similar to HIV. The fusion proteins were a way to spread the infection to other cells by simply merging them with the infected one (HIV does this too).