Microorganism-host relationship;

In, nature, microorganisms exist in harmony with their hosts co-evolve, the hosts progressively become resistant to the microorganisms. When a microbe jumps from a long-standing animal host to human beings, it may cease to be a harmless parasite and become pathogenic. To survive, a pathogen has to be a chronic infection, stay alive in the host for a long time or have a non-human reservoir in which to live while waiting for new hosts to pass by. In fact, the human is actually an accidental victim and a deal-end host for many human diseases like rabies, anthrax, tularemia, West Nile virus and many others.
Most of the modern infectious diseases emerged as zoonotic infections. It is difficult to determine which diseases jumped from animal to humans, but there is good evidence that measles, smallpox, influenza, HIV and diphtheria came to humans by microbes jumping from one species to the other. H5N1 is a strain of avian influenza / bird flu but there is evidence of its attack on jumped from civet of humans probably because of inadequate cooking of civet meat for human consumption. Similarly, West Nile virus has spilled over into human populations probably due to interactions between the carrier host and domestic animals. This virus is part of the Japanese encephalitis antigenic complex of viruses. It mainly infects birds. However, cases of infection by this virus have also been reported in humans and domestic animals. The main route of human infection is through the bite of an infected mosquito.