| Word | Meaning |
|---|---|
| virus | An infectious agent that replicates only inside a living cell. |
| immune system | The body's defence system against infection and disease. |
| antibody | A protein produced by the body to neutralise bacteria or viruses. |
| pathogen | A bacterium, virus or other microorganism that can cause disease. |
| microscopic | Too small to be seen with the naked eye. |