Luis Enjuanes
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Viruses are an integral part of the ecosystem, they exist wherever there is life, and are the most abundant living beings in the planet. As a matter of fact their biomass is as abundant as the biomass of prokaryotes. Viruses are very abundant in marine waters, exceeding a million particles every millilitre. There are around 1030 viruses in the sea in total. If we queued, all viruses present in the sea would compose a line long enough to connect the 60 closest galaxies. Some of the viruses that have infected humans are associated to the first precursors of mammals and have coevolved with us. Other viruses have entered human populations in recent times. In the last 10.000 years of human history, dramatic changes have affected both humans and the viruses that infect us. Many animals have been domesticated, human population, as well as travelling, have increased drastically. Some viruses were transmitted from animals to humans as it occurs nowadays. Virulent viruses like measles and smallpox have reached a point of equilibrium with mankind in recent times. These viruses would either kill their host or provide them with long-term immunity. Thus, these viruses could only survive in large human populations with possible hosts always available for propagation. The less virulent viruses established a long-term relationship with their hosts; this implies that probably they were the initial ones to adapt to first human populations. These viruses include retroviruses (endogenous human viruses), herpesviruses and papillomaviruses.

The barriers between human and animal infections are very small and viruses constantly cross these barriers. Almost two thirds of the total transmissible human diseases are zoonosis, which means that they are acquired from vertebrate animals. Similarly, around 75% of emergent human diseases are also zoonosis, and many are food related. The flu, the acquired immune deficiency syndrome (HIV/AIDS), the severe acute respiratory syndrome cause by the SARS-CoV virus, the West Nile virus, salmonellosis, spongiform encephalopathy, trichinosis and brucellosis are all human infection transmitted from animal to man.

The routes and efficiency of viruses transmitting between humans are very variable. Putting it simply, transmission occurs through contact (oral-faeces), air, and blood, with the help of vectors like mosquitoes and ticks, or vertically from parents to sons. There are high mortality viruses like the Hantavirus, Ebola or Zaire that cause haemorrhagic fevers, which fortunately can only be transmitted through contact with infected patients. But there are other low-mortality viruses like the flu or SARS that can be transmitted through the air associated to small particles. Insects like the mosquitoes also play a key role in the transmission of viruses. This is the case  of the virus that causes yellow fever.

Viruses interact with hosts in very different ways. Both battle each other using a complex arsenal of resources to try to keep the other one under control. These resources are part of the genetic characteristics of the virus and the organism’s cells. The later use different lines of defense to prevent the viruses from infecting.

The occurrence of epidemics has been constant along human history and will probably continue in the future. In 1930 Scrapie was described, a new disease affecting lambs that had to be caused by filterable agent such as virus, or an even smaller organism. In 1960, Carlton Gajduseck (US National Institute of Health, NIH) described Kuru, a disease similar to Scrapie in humans, which caused tremors, loss of equilibrium and speech, and finally death. This disease was analogue to Creutzfeldt–Jakob syndrome caused by a genetic anomaly.

During the War of Korea (1951-1953) a haemorrhagic fever affected 2.000 United Nations soldiers. Its transmissible nature was registered after testing that the disease could disseminate through the serum and urine of infected mice. The epidemiologic evidence suggested that wild mice where the carriers of the virus producing the diseases. In 1976 the virus was finally identified in mice and it turned out to be the Hantavirus that received the name of the Hantaan river in Korea, the first place where it was identified. Until the present, it has been detected in Japan, Russia, Sweden, Finland, France, USA and South America, which include countries very close to Spain. This virus normally kills half of the people that get inffected.

Bats are animals infected by a wide variety of viruses that are later transmitted to human populations. It is the case of the Nipah virus or SARS-CoV virus among other.
The West Nile virus remerged in New York in 1999, a location were it never had been detected before. The emerging strain was related to another one previously detected in Israel. After that, the virus has extended through the USA, Canada, Mexico, and the Caribbean. In October 2006, 31 cases had been confirmed and 25 more were probable only in New York, and caused 7 deaths. The virus was isolated in birds living in parks in the areas of New Jersey and Connecticut. The genes of the virus obtained from birds were exactly the same as the virus affecting humans. The virus is transmitted by mosquitoes and affects many domestic animals, including horses and birds. The first time it was isolated was in a province of the West Nile, in Uganda, in 1937, in the blood of a woman suffering fever; although the largest infection was recorded in 1974 in South Africa. This virus is currently very spread around the World.
   Recently, we are seeing how old infectious diseases like tuberculosis, malaria or Dengue are starting to reappear. Also new mortal haemorrhagic diseases caused by viruses have developed in South America and Africa. A collection of infectious mortal diseases caused by viruses receiving exotic names such as Lassa, Rift Valley, Semliki Forest virus, Crimea-Congo, Marburg, Ebola, Japanese encephalopathy, etc., which are extending towards Occident. The chikungunya virus transmitted by mosquitoes has had an explosive dissemination in countries around the Indian Ocean. In the French island or Reunion approximately 1 of every 10 inhabitants have been affected, and the virus has already reached Mauritius and Seychelles.

It is estimated that currently the Hepatitis C virus affects around 170 million people worldwide, and it is expected that in a near future it will further extend in a alarming way. This virus causes persistent infections and liver cancer and there is no vaccine to control its dissemination. In November 2002 the SARS caused by a coronavirus has infected more than 8000 people causing a 10% of deaths. Zoonosis is a huge danger for human populations.

All the data put together suggest that due to the emerging diseases and reappearance of other already known, it will be necessary to work on developing new vaccines against them. And this need is further reinforced due to the current development of biological weapons in at least a dozen countries, including Russia and USA, as well as due to the possible activation of bioterrorism.

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