How Do Viruses Work?

This is one of biology's genuinely open questions. Viruses do not eat, breathe, grow, move, or reproduce on their own. Outside a host cell they are essentially inert chemical packages. Most biologists place them in a category of their own, not quite alive, not quite not.

What viruses do have is genetic information, either DNA or RNA, wrapped in a protein shell called a capsid, and sometimes an outer envelope stolen from the last cell they infected. That genetic information is the entire instruction set for their own replication. They are blueprints looking for a factory.

The question matters more than it might seem. If viruses are not alive, they cannot be killed, only dismantled or blocked. This is why antiviral drugs work very differently from antibiotics, and why the same approach cannot be used for both.

Bacteria are single-celled living organisms. They have their own metabolism, reproduce independently, and can be killed by antibiotics that disrupt their cell processes. A bacterial infection is an invasion by an independent living organism.

Viruses are not cells and have no independent metabolism. They are molecular instructions that can only operate inside a host cell. Antibiotics have no effect on viruses because viruses do not have the cellular structures that antibiotics target.

This distinction matters enormously in medicine. Taking antibiotics for a viral illness like the flu or a cold does nothing to treat the infection and contributes to antibiotic resistance, one of the most serious public health problems of the current era.

Vaccines work by showing the immune system a harmless version of a threat before the real one arrives. This can be a weakened or killed virus, a fragment of the virus's protein coat, or, in the case of mRNA vaccines, instructions for your own cells to briefly produce a viral protein.

The immune system responds to this preview by producing antibodies and creating memory cells. These memory cells persist for years or decades. When the actual virus arrives later, the immune system recognises it immediately and launches a rapid response before the infection can take hold.

This is not new technology. The principle of vaccination was understood before germ theory existed. Edward Jenner observed in 1796 that milkmaids who caught cowpox did not get smallpox, and used this observation to create the first vaccine. The disease smallpox caused was subsequently eradicated entirely from the planet in 1980, the only human infectious disease ever to achieve this.

Viruses mutate because copying genetic material is never perfectly accurate. Every time a virus replicates, there is a small chance of an error in the copy. Most errors are harmless or lethal to the virus. Occasionally, an error produces a variant that is better at infecting cells, evading the immune system, or spreading between hosts.

RNA viruses mutate particularly fast because their copying enzymes lack the proofreading mechanisms that DNA copying enzymes have. This is why influenza, HIV, and coronaviruses change so rapidly.

Evolution favours viruses that spread efficiently. A virus that kills its host too quickly spreads less far. This is why highly lethal viruses tend to be less successful in the long run than milder ones that allow the host to move around and transmit them further.

By the late 1800s, scientists had established that many diseases were caused by bacteria, which could be filtered out of liquid using fine porcelain filters. In 1892, Russian botanist Dmitri Ivanovsky was investigating a disease that devastated tobacco plants. He filtered infected plant sap, expecting to remove all pathogens. The filtered sap still caused disease.

Six years later, Dutch microbiologist Martinus Beijerinck repeated the experiment and concluded that the infectious agent must be something fundamentally different from bacteria. He called it a contagium vivum fluidum, a living contagious fluid. It could pass through filters that stopped bacteria. It could not be grown in a dish. It could only replicate inside living cells.

Beijerinck had discovered viruses without being able to see one. Viruses were not actually visualised until electron microscopes became available in the 1930s. What Ivanovsky and Beijerinck had found was an entirely new category of biological entity, and neither fully grasped what they had done.