Hemorrhagic fever diseases such as yellow fever, dengue fever, and ebola are caused by whereas nonhemorrhagic fever diseases such as brucellosis, q fever, and cat-scratch fever have as the causative agents of disease.

I would say that hemorrhagic fever diseases such as yellow fever, dengue fever, and ebola are caused by viruses, whereas nonhemorrhagic fever diseases such as brucellosis, q fever, and cat-scratch fever are caused by bacteria. Diseases caused by bacteria can often be treated with an antibiotic, whereas the treatment of viruses is more problematic, and many diseases caused by viruses cannot be cured through medication, including ebola.