Viruses and virology

A virus is a microscopic infectious agent that can reproduce only inside the living cell of a host. A virus particle consists of genetic material — either DNA or RNA — enclosed in a protein coat called a capsid, and in many cases an outer lipid envelope. Because viruses lack the machinery to make proteins or generate energy on their own, they are not considered cells and occupy a debated position at the boundary of what counts as living. Virology is the branch of microbiology that studies them.

Infection begins when a virus attaches to a specific molecule on the surface of a target cell and enters it. The virus then uses the cell's own machinery to copy its genetic material and assemble new particles, which are released to infect further cells, often damaging or killing the host cell in the process. HIV, the virus that causes AIDS, is a retrovirus that copies its RNA into DNA and attacks the immune system itself. Other well-known viruses include influenza, which mutates rapidly and causes seasonal epidemics; ebola, which causes severe haemorrhagic fever; and the herpes and adenovirus families. A few viruses are also linked to certain cancers.

The body defends itself through the immune system, which produces antibodies and other immune cells targeted at a specific virus. Vaccines exploit this by presenting a harmless version or fragment of a virus so that immunity develops before any real exposure. Antiviral drugs can slow some infections by blocking steps in viral replication, but unlike antibiotics, which act on bacteria, they have no effect on bacterial disease.

Frequently asked questions

Are viruses alive?

It is debated. Viruses carry genetic material and evolve, but they cannot reproduce or make energy without hijacking a living host cell, so they are not classed as cells.

What makes HIV a retrovirus?

HIV stores its genes as RNA and uses an enzyme to copy that RNA into DNA inside the host cell, the reverse of the usual flow, which is the defining feature of retroviruses.

Why don't antibiotics cure viral infections?

Antibiotics target structures and processes specific to bacteria. Viruses replicate using host cells and lack those targets, so antivirals or vaccines are needed instead.