Humans viruses
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Most viruses are too small to be seen directly with a light microscope. Viruses infect all types of organisms, from animals and plants to bacteria and archaea. Since the initial discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898 about 5,000 viruses have been described in detail, although there are millions of different types. Viruses are found in almost every ecosystem on Earth and are the most abundant type of biological entity.
On the question of whether virus are alive, it seems viruses straddle the definition of life. They lie somewhere between supra molecular complexes and very simple biological entities. Viruses contain some of the structures and exhibit some of the activities that are common to organic life, but they are missing many of the others.
In general, viruses are entirely composed of a single strand of genetic information encased within a protein capsule. Viruses lack most of the internal structure and machinery which characterize 'life', including the biosynthetic machinery that is necessary for reproduction. In order for a virus to replicate, therefore, it must infect a suitable host cell.
And we learned in biology viruses exist in two distinct states. When not in contact with a host cell, the virus remains entirely dormant. During this time there are no internal biological activities occurring within the virus, and in essence the virus is no more than a static organic particle.
In this simple, clearly non-living state viruses are referred to as virions. These can remain in this dormant state for extended periods of time, waiting patiently to come into contact with the appropriate host. When the virions comes into contact with the appropriate host, they become active and are then referred to as a virus. It now displays properties typified by living organisms, such as reacting to its environment and directing its efforts toward self-replication. Virus can cause infections ranging from the more innocuous, such as a cold, to the more malevolent, such as hemorrhagic fever or rabies.
An increasing number of antiviral remedies are being developed that prevent the virus multiplying and help cause the illness to run its course more rapidly. Treating human viruses will depend on the strength of the individual’s immune system, their overall health status, age, the severity of the condition, and the type of viruses involved.
Minor illnesses caused by viral infections usually only require symptomatic treatment (such as painkillers and anti-inflammatories) while more severe conditions may require advanced medical treatment and sometimes even life-long treatment. Antiretroviral therapy suppresses the replication of the human immunodeficiency virus (HIV), even if there are no symptoms. The aim of treatment is to lower the concentration of virus (viral load).
Animal to Human Viruses
The study of animal viruses is important for a number of reasons. Many animal viruses are also important from a human medical perspective. The emergence of the SARS virus in the human population, coming from an animal source, highlights the importance of animals in bearing infectious agents; avian influenza viruses can directly infect humans.
Generally, rising food prices is under developed countries where people are unable to afford basic supplies, has resulted some communities in Central Africa, for example to increasingly turn to the forests for food. In doing so, hunters expose themselves to hidden dangers - microscopic pathogens living in the blood of forest animals.
Whilst many of these viruses are harmless, some are potentially deadly when passed to humans. Scientists point out there's nothing new about these viruses. What is new, however, is the frequency of people's contact with them and how easily they can now be spread around the world.
Epidemiologist Dr. Nathan Wolfe is following the hunters. "Individuals have been infected with these viruses forever," Wolfe said. "What's changed, though, is in the past you had smaller human populations; viruses would infect them and go extinct. Viruses actually need population density as fuel."
Wolfe works mostly in the forests of Cameroon tracking these viruses that can jump from animals to humans - what are called Zoonotic viruses. Of the 1415 pathogens known to affect humans, 61% are Zoonotic.
The most prolific and deadly Zoonotic is HIV, the virus that causes AIDS. In 1999, scientists at the University of Alabama at Birmingham traced the origins of HIV back to a subspecies of chimpanzee. The virus might have jumped to humans when the blood of an infected chimpanzee came in contact with the blood of a bush meat hunter during the killing or butchering of the animal.
It took decades, but that simple, seemingly insignificant transmission set off a global epidemic, or pandemic, that so far has killed or infected tens of millions of people. HIV probably crossed into humans as far back as the early 1900s, but it wasn't until air travel became common that the virus spread, and AIDS became a global epidemic in the 1980s.
Computer Virus
A computer virus is a computer program that can copy itself and infect a computer. It’s important to distinguish a computer virus from other malware programs such as adware and spyware that do not have the reproductive ability.
A computer virus can spread from one computer to another (in some form of executable code) when its host is transferred from one computer over a network, the Internet, or even carried using a removable medium such as a CD or USB. The effectiveness of these viruses (to spread) is increased when they use computer hosts that are commonly accessed by other systems.
Complex viruses
A polymorphic virus is a virus that changes its appearance in host programs. For instance, it encrypts its body with a different key each time, and prepends a decryption routine to itself. The decryption routine (known as the "decryptor") is mutated randomly across virus instances, so as to be not easily recognizable.
A metamorphic virus, by comparison, is a virus that also changes its appearance in host programs, however it does so without necessarily depending on encryption. The difference in appearance comes from changes made by the virus to its own body such as through the insertion and removal of "garbage" instructions and switching between two different opcodes that are functionally-equivalent making analysis and detection difficult.
Examples of computer viruses