Why do Viruses Kill?
Pandemic Viruses and Biological Weapons

Why do Viruses Kill?
Pandemic Viruses and Biological Weapons

A pandemic is a worldwide epidemic. The 1918 flu pandemic, commonly referred to as the Spanish flu, was a category 5 influenza pandemic caused by an unusually severe and deadly influenza A virus.

The victims were often healthy young adults, in contrast to most influenza outbreaks, which predominantly affect juvenile, elderly, or otherwise-weakened patients.

Just months ago, the world stood in fear of an emerging new disease that threatened to kill millions. A new flu variant H1N1 had arrived. In the UK alone, 65,000 deaths were predicted.

Yet to date, these dire warnings have not materialized.

If this latest pandemic has taught anything, it is just how little is understood about the invisible world of viruses. But that has not stopped scientists trying.

Horizon follows the leading researchers from across the world, who are attempting to unravel the many secrets of viruses to understand when and why they kill.

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 Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants, and the 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. The study of viruses is known as virology, a sub-speciality of microbiology.

Common human diseases caused by viruses include the common cold, the flu, chickenpox and cold sores. Serious diseases such as Ebola and AIDS are also caused by viruses.

Many viruses cause little or no disease and are said to be "benign". The more harmful viruses are described as virulent. Viruses cause different diseases depending on the types of cell that they infect.

Some viruses can cause life-long or chronic infections where the viruses continue to reproduce in the body despite the host's defence mechanisms. This is common in hepatitis B virus and hepatitis C virus infections.

People chronically infected with a virus are known as carriers. They serve as important reservoirs of the virus. If there is a high proportion of carriers in a given population, a disease is said to be endemic.

There are many ways in which viruses spread from host to host but each species of virus uses only one or two. Many viruses that infect plants are carried by organisms; such organisms are called vectors. Some viruses that infect animals and humans are also spread by vectors, usually blood-sucking insects.

However, direct animal-to-animal, person-to-person or animal-to-person transmission is more common. It is important to know how each different kind of virus is spread to prevent infections and epidemics.

Doctors Of Death

Biological warfare — also known as germ warfare — is the deliberate use of disease-causing biological agents such as bacteria, viruses, fungi, or biological toxins, to kill or incapacitate humans, animals or plants as an act of war.

Biological weapons (often termed "bio-weapons" or "bio-agents") are living organisms or replicating entities (viruses) that reproduce or replicate within their host victims.

The ability of viruses to cause devastating epidemics in human societies has led to the concern that viruses could be weaponised for biological warfare.

Further concern was raised by the successful recreation of the infamous 1918 influenza virus in a laboratory. The smallpox virus devastated numerous societies throughout history before its eradication.

There are officially only two centers in the world that keep stocks of smallpox virus – the Russian Vector laboratory, and the United States Centers for Disease Control.

But fears that it may be used as a weapon are not totally unfounded; the vaccine for smallpox has sometimes severe side-effects – during the last years before the eradication of smallpox disease more people became seriously ill as a result of vaccination than did people from smallpox – and smallpox vaccination is no longer universally practiced.

Thus, much of the modern human population has almost no established resistance to smallpox.