The emergence of new viruses is related to the viral evolution, the ecological change of niche, the modification of the environment, the bioterrorism…
The viruses have intrinsic capacities of evolution.
- viral productivity . HIV produces 109 virus/day.
- viral diversity: changes ata rate of 10-4 for a virus with ARN and 10-9 for the viruses with ADN.
- Recombinations : recombining derive from a common ancestor.
- Genetic restocking.
Modification of the environment
- Climatic changes can act on the influence of the vectors which are the mosquitos (or ticks, rodents). Each virus has a particular vector.
- Changes of way of life: urbanization in the shantytowns where there are bad conditions of hygiene (water used in the tires where there are many mosquitos).
- Intercontinental voyages men and livestock.
- Deforestation (South America) : the men return in contact with the virus of the monkeys.
- The increase in hygiene. With XIXth century, the children met the virus of the polio as of the birth thus they were immunized. To the XXth century, the viruses of the environment are in less large quantity thus them men meet the virus later and are not protected any more by maternal antibodies.
Ecological change of niche
Infection of a native population by a virus resulting from the same species
- variola where survivors contracted measles
- yellow fever like colonists in Philadelphia 1793
- the construction of Panama Canal
Infection by a virus resulting from another species
For example rotavirus; the calcicivirus in the marine mammals, gives acute diarrhoeas at the Man
1999: Hendra virus in Malaysia one gives hemorrhagic encephalitis or West Nile in New York.
2000: Lassa virus, two cases imported from the Ivory Coast to Hamburg and Sierra Leone to London, Dengue