Viruses

A virus is a microscopic infectious agent that can replicate only inside the living cells of a host organism. A virus particle consists of genetic material, either DNA or RNA, enclosed in a protein coat called a capsid; some types also have an outer lipid envelope. Because they lack the machinery to grow or reproduce on their own, viruses are not usually classified as living organisms in the same sense as bacteria, plants or animals.

To multiply, a virus attaches to a suitable host cell, inserts its genetic material, and hijacks the cell's own systems to produce new virus particles. The newly made viruses are then released to infect further cells. Different viruses target different hosts and tissues, which is why a given virus typically causes a specific set of symptoms. Human viral diseases range from common, mild infections such as influenza and the common cold to severe illnesses such as smallpox, polio, ebola and the diseases caused by avian influenza strains such as H5N1.

The body defends against viruses through the immune system, which can recognise and neutralise viral particles and infected cells. Vaccines exploit this by exposing the immune system to a harmless form or fragment of a virus, training it to respond rapidly to a real infection. Vaccination campaigns have controlled or eliminated several once-common viral diseases, including the global eradication of smallpox. Antiviral drugs can also help treat some infections, although they work differently from antibiotics, which act only on bacteria.

Frequently asked questions

Are viruses alive?

Viruses occupy a grey area. They carry genetic material and evolve, but they cannot reproduce or carry out metabolism without hijacking a living host cell, so they are often described as being on the boundary of life.

Why don't antibiotics work against viruses?

Antibiotics target structures and processes specific to bacteria, which viruses do not have. Viral infections require antiviral drugs, vaccines or the body's own immune response.

What is H5N1?

H5N1 is a subtype of avian influenza virus, commonly called bird flu, that mainly infects birds but can occasionally infect humans and cause serious illness.