These are two forms of virus. My guess would be that they were isolated and named separately, and retained their original names even after it was discovered the

These are two forms of virus. My guess would be that they were isolated and named separately, and retained their original names even after it was discovered they were the "same" virus. The mechanism sounds similar to what happens with HIV. Simplifying wildly, there are M-tropic strains that infect macrophages, and T-tropic strains that infect T lymphocytes. People tend to get infected with M-tropic strains, because it's macrophages that come into contact with invading viruses. However, over time, the virus can mutate to become T-tropic. It's the T-tropic strains that cause immunodeficiency, and hence AIDS.