Viruses

Viruses are extremely tiny germs that are acellular (not made of cells) and can reproduce only inside a living host cell.

• What they are made of: A virus is basically genetic material (DNA or RNA) wrapped in a protein coat. Some viruses also have an outer fatty envelope.
• Why they need a host: Viruses do not have the cell machinery needed to make proteins or copy their genes. So they “hijack” a host cell and use the cell’s machinery to make many new viruses.
• How infection spreads: After many new viruses are made, they usually burst out (or leave) the host cell and go on to infect more cells.
• Who they can infect: Different viruses infect different hosts—plants, animals, and even bacteria. Viruses that infect bacteria are called bacteriophages.
• Everyday examples: Common cold, flu, chickenpox, dengue, and COVID-19 are caused by viruses; some plant viruses can cause yellowing or curling of leaves.
• A useful way to think about them: A virus is like a “set of instructions” in a tough cover—it becomes active only when it gets inside the right living cell.