More virus variations than stars in the milky way

Why is it difficult to study viruses? They just keep changing! They can mutate and evolve rapidly compared to anything else, leading to numerous variations of any one virus. Check out this quote about the possible virus variants from Dr. Vincent Rancaniello, virologist at Columbia University Medical Center.

"The answer is quite simple: for a genome N nucleotides in length, there are 4^N possible variants, because there are 4 different nucleotides. This is a huge number, even for small viral genomes. For example, there are 10¹⁸⁰ different variants for a genome that is only 300 nucleotides in length. The HIV-1 genome, which is about 10,000 nucleotides in length, can exist as 10⁶⁰²⁰ different sequences. To put this number in perspective, there are 10¹¹ stars in the Milky Way galaxy and 10⁸⁰ protons in the universe."