ABSTRACT
Hemorrhagic fevers have a diverse etiology and those of an infective origin may be classified according to the etiological agent responsible for their development. It is known that the hemorrhagic state may result from the circulation in the bloodstream of viruses, chlamydia, rickettsiae, bacteria, fungi, spirochaetes, and protozoa, and even, though rarely, helminths. Although the hemorrhagic state may develop in a variety of infections, the term hemorrhagic fever has acquired a more specific meaning, referring to certain arthropod-borne and certain rodent-borne infections. The arthropod-borne infections have been classified according to the antigenic composition of the viruses based on the results of hemagglutination inhibition, complement fixation, and neutralization tests. Transmission to vertebrates is usually by arthropod bite, but aerosol infection has been demonstrated in Rift Valley fever and transstadial transmission in Crimean-Congo hemorrhagic fever. Transovarial transmission of some species occurs in arthropods and transplacental transmission occurs in some mammals.
