Viruses
D. Indumathi, The Institute of Mathematical Sciences, Chennai

Viruses are found wherever there is life and have probably existed since living cells first evolved. A virus is a small infectious agent that can replicate only inside the living cells of an organism. Viruses can infect all types of organisms, animals and plants and even bacteria.
Virus particles (known as virions) are made up of two or three parts: i) the genetic material made from either DNA or RNA, which are long molecules that carry genetic information; ii) a protein coat that protects these genes; and in some cases iii) an envelope of lipids (fat) that surrounds the protein coat when they are outside a cell.
The shapes of viruses range from simple helical and icosahedral forms to more complex structures. The average virus is about one one-hundredth the size of the average bacterium. Most viruses are too small to be seen directly even with a powerful optical microscope.
There are millions of different types of viruses. Shown are some typical major types, including a bacteriaphage, or "bacteria-eater". Also shown is a computer-aided reconstruction of a rotavirus. It is the most common cause of severe diarrhoea among young children.
Viruses are considered by some to be a life form, because they carry genetic material, reproduce, and evolve through natural selection. However they lack key characteristics (such as cell structure) that are generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life".
For the virus to multiply it must enter cells of the host organism and use the host material to reproduce. To enter the cells, proteins on the surface of the virus interact with proteins of the cell. Attachment, or adsorption, occurs between the viral particle and the host cell membrane. A hole forms in the cell membrane, then the virus particle or its genetic contents are released into the host cell.
Next, a virus must take control of the host cell's replication mechanisms. After control is established and the environment is set for the virus to begin making copies of itself, replication occurs quickly by the million.After a virus has made many copies of itself, it usually has exhausted the cell of its resources. The host cell is now no longer useful to the virus, therefore the cell often dies and the newly produced viruses must find a new host. The process by which virus progeny are released to find new hosts, is called shedding. This is the final stage in the viral life cyle.

Life cycle of a flu virus
The H5N1 “avian” flu is highly deadly among birds, and it has occurred in humans. Most people who contract the avian flu have had direct contact with infected birds. Fortunately, H5N1 does not appear to spread from human to human, but what if mutations occur in the virus that allow this to happen?
The 2009 H1N1 “swine” flu reminded us of the potential for flu pandemics that cross over to humans from other animals.
When an unsuspecting victim inhales a flu virus, a protein on the surface of the virus called hemagglutinin (the ‘H’ in H1N1, for example) binds to a receptor called sialic acid on the surface of cells in the respiratory tract.
The cell takes in the virus, which then replicates inside the host cell. The newly formed viruses leave the cell by budding off the surface. The hemagglutinin on the new viruses can then bind to the sialic acid on the surface of other host cells, thus infecting new cells. For this to occur, however, the viruses need to escape the host cells. Another viral surface protein, neuraminidase (the ‘N’ in H1N1, for example), breaks down the sialic acid receptors so the viruses can escape.
Medicines such as Tamiflu and Relenza stop the multiplication of the virus because they are structurally similar to sialic acid and will bind to the neuraminidase, but they cannot be broken down. Therefore, neuraminidase cannot bind to sialic acid on the cell surface when it has Tamiflu or Relenza bound to it. The two drugs are collectively referred to as neuraminidase inhibitors (NAIs). When NAIs are present, some of the newly produced virus will get stuck to the original host cell and will not be able to infect other cells. Some viruses will still escape, so NAIs don’t cure the flu but rather reduce its severity.

Bacteria versus viruses
Since ancient times, reports of river waters having the ability to cure infectious diseases such as leprosy have been documented. In 1896, Ernest Hanbury Hankin reported that something in the waters of the Ganges and Yamuna rivers in India had marked antibacterial action against cholera and could pass through a very fine porcelain filter. In 1915, British bacteriologist Frederick Twort, superintendent of the Brown Institution of London, discovered a small agent that infected and killed bacteria. d'Hérelle, working at the Pasteur Institute in Paris, announced on 3 September 1917, that he had discovered "an invisible, antagonistic microbe of the dysentery bacillus". These are bacteriophages, that have immense use in killing bacterial infections by literally killing off the bacteria from within itself. A diagram of how some bacteriophages infect bacterial cells is shown in the figure, which is not to scale; bacteriophages are much smaller than bacteria. 
Today, the world is watching to see if anti-biotics continue to be effective against all forms of infections and whether the potential of viruses can be tapped to fight disease.