Most diseases in the world are bacterial or viral in origin. A few others have fungal origin but these are restricted to minor skin infections and are curable. Bacterial diseases though chronic in nature are treatable due to recent advancements in medicines and their delivery. Viral diseases are tough to cure as 1.symptoms for the disease varies from person to person 2.very sensitive tests is required for their detection. For instance a person tested negative for ELISA may be tested positive for WESTERN BLOT which is the final confirmatory test for detection of HIV. Viral cycles are indefinite and depend on the environment they reside. Most viruses are avirulent outside human body. A few have evolved to live invitro and sometimes are more pathogenic than invivo. Rabies virus is a testimony to the above statement.

Rabies belongs to the class of diseases caused by Rhabdoviridae family. It belongs to genus Lyssavirus. It is a retrovirus i.e. virus having RNA as the genetic material. The virus has helical symmetry and a diameter of approximately 73nm.High concentration of virus is found in the saliva of mammals. Mammalian saliva and tears have high amounts of lysozyme and RNase. These can cleave bacterial cell wall and hence are antibacterial. But the virus thrives in this environment due to the outer coat called capsid. The viral capsid protein is extremely resistant.

Rabies infection is primarily caused due to the bite from an infected animal. Other modes of transmission include entry via open wound, burns and cuts in the epidermis. Trace amounts of virus is also found in other body fluids like feces, mucous, sweat and genital secretions. Transmission is also possible when we come in contact with these fluids just like a Leptospirosis (Rat fever). The disease affects neural pathways and causes damage to the central nervous system (CNS). If untreated it may lead to Encephalitis (inflammation of brain).

Mammalian sources of rabies include Dogs, Skunks and Bats. The nature and pathogenecity of the virus existing in each is different. Sometimes they differ between 2 species living in different environment, for instance a dog in Poland and in India may carry different forms of the virus which may vary in their virulence.

Transfer of blood from infected person is also a major cause of this disease. This is because most of the blood screening techniques are sensitive only to bacteria and few viruses. Cost is a major factor here and almost all sensitive tests are quiet costly. Blood screened for viruses may also be ineffective as the tests rely on a critical concentration of viruses. When a particular virus happens to be in a concentration less than the critical concentration then they are not detected.

Mammals have evolved various graft rejection mechanism. Graft refers to a section of tissue being transferred. A graft between 2 individuals of the same species is called as an allograft. An allograft from an infected person may cause viral transfer. This is possible in places where facilities for graft screening are meager.