Written by Maciamo
Japanese is a heavily hybridized language. Roughly half of all words in Japanese were borrowed from Chinese (most through kanji compounds). The Japanese started importing words from European languages since they first came into contact with them. It began with the import of Portuguese words in the 16th century, then Dutch ones during the Edo period (1600-1867), when the Dutch were the only Westerners tolerated to trade with Japan.
From the Meiji Restoration (1868), a great deal of German, English and French words entered the Japanese language. Sometimes words in these languages sounded alike. For example a word like 'virus' is even spelled the same in English, German, French and Latin, but has a different pronuciation in each language. In this particular case, because the word was imported both from Latin and from German, it was rendered in Japanese as ウイルス and ビールス, which is why the Japanese doesn't sound at all like the English one (which would have been ヴァイラス in Japanese).
Many native English speakers often have trouble understanding "Katakana English", because Japanese sometimes alter the original pronuciation (well, most words actually), shorten the word (e.g. "remokon" for "remote control") or change its original meaning (eg "maibu-mu" from "my boom" means something "trendy" as in "This skirt is my boom"). Better be warned before setting foot in Japan.
It is a common belief among English speakers to think that all katakana words that do not sound like in English are mistranslations from English. Actually, in many cases, those katakana words just do not come from English at all. That did not prevent them from being deformed from their original language anyway, as in zubon (trousers/pants), a semantic and phonetic corruption from the French jupon (meaning 'skirt').
Interestingly, some foreign words have acquired a kanji spelling since their adoption (eg. tempura 天麩羅, kan 缶).
Here is a list of imported words in Japanese that do not come from English. Chinese words are of course excluded, as there would be too many of them.