Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
infectious [ɪnˈfɛkʃəs] adj (person, animal) зара́зный*зара́зен (disease) инфекцио́нный; (fig) зарази́тельный*зарази́телен
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: