Viruses and bacteria

Microbes are organisms or particles too small to see without a microscope. Bacteria and viruses are the most familiar, and both are often called germs, but they are fundamentally different kinds of things.

Antonie van Leeuwenhoek, a Dutch draper who made his own microscopes, was the first to describe bacteria in the 1670s. Viruses were not identified until the 1890s, when scientists found that the agent of a tobacco plant disease could pass through filters fine enough to trap bacteria.

Bacteria are single-celled living organisms. They have their own metabolism, make their own proteins and reproduce by dividing in two, sometimes as often as every 20 minutes. Their DNA floats in the cell rather than sitting inside a nucleus.

Bacteria live almost everywhere, from soil and oceans to hot springs and the human body. The human gut hosts trillions of them, and most bacteria are harmless or helpful: they digest food, make vitamins, fix nitrogen for plants and break down dead matter.

Viruses are far smaller than bacteria and are not cells. A virus is essentially genetic material, DNA or RNA, packaged in a protein coat, sometimes with an outer envelope. It has no metabolism and cannot reproduce on its own.

To multiply, a virus must enter a living cell and hijack the cell's machinery to make copies of itself. Because of this, scientists debate whether viruses should be considered alive. Viruses infect every form of life, including bacteria, which are attacked by viruses called bacteriophages.

• Some bacteria release toxins, as in cholera, tetanus and botulism.
• Bacterial infections include strep throat, tuberculosis, many urinary tract infections and some pneumonias.
• Viruses damage or destroy the cells they infect and trigger immune responses that cause symptoms.
• Viral diseases include colds, influenza, measles, HIV and COVID-19.
• Only a small fraction of bacteria cause disease, while many viruses are specialised to particular hosts.

Antibiotics work by attacking structures and processes that bacteria have but human cells do not, such as bacterial cell walls and ribosomes. Viruses have none of these, which is why antibiotics do nothing against colds or flu. Taking them for viral infections only encourages antibiotic resistance.

Antiviral drugs exist for some viruses, such as HIV, influenza and hepatitis C, and usually block a specific step in the virus's life cycle. For many viral infections, the best protection is prevention through vaccines, hygiene and, when ill, rest while the immune system clears the infection.

People harness bacteria to make yogurt, cheese and vinegar, to treat sewage and to produce medicines. Genetically modified bacteria have produced human insulin since the early 1980s, replacing insulin taken from animals.

Viruses are useful too. Modified viruses are used as vectors to deliver genes in some gene therapies and vaccines, and bacteriophages are being studied as a possible treatment for antibiotic-resistant infections.