Although the word “virus” has become a part of the everyday vernacular–what exactly is a retrovirus? Marc Johnson says viruses can be grouped into RNA viruses and DNA viruses. RNA viruses cause short-term diseases such as the flu and the common cold, whereas DNA viruses cause more long-term illnesses like herpes or cancer. “Retroviruses are a unique blend,” he explains. “They are like a DNA virus that can replicate with RNA strategies.” HIV is a retrovirus.