What is the rule for compound words in English? I know that this is pretty common in German but for English, I find it irregular in terms of the compound words.
Names of viruses are often formed as single words with the suffix -virus. This is perhaps influenced by scientific use. There are rhinoviruses, noroviruses, adenovirus and so on. Again there isn't a logical rule, that is just how it is done. The virus that causes Ebola is known as ebolavirus (and note the downcasing when Ebola becomes a prefix) — ELL
Also, have a look at this Wikipedia article, almost all the names of viruses are one word.
For viruses that begin with a Greek letter, write it out and close up space between the letter and the rest of the word — CDC web
Edit: Suffix -virus: Used to form taxonomic names of genera of viruses — Wiktionary