From the human point of view, some viruses can be useful, potentially useful, or at least interesting.
Under the “interesting” heading would fall the mosaic virus, which produced the fantastically variegated tulips of the tulipomania investing craze in seventeenth-century Holland.
The rare infected bulbs were treated almost like a currency with a constantly multiplying value until the market crashed in 1637. It was discovered that the infection weakened the plants, and now hybridizers work to create the same colorful effects by breeding rather than infection.
Unquestionably useful in the field of public health is the vaccinia virus. It was discovered that milkmaids infected with relatively mild cowpox were immune to virulent smallpox, and the knowledge eventually led to widespread vaccination, eradicating smallpox as a disease. Later, the related vaccinia virus was used for this purpose.
In genetic medicine experiments, harmless viruses are used to carry desirable genes into cells.
For example, scientists have tried treating a genetic disease called familial hypercholesterolemia by letting a virus insert a crucial gene into liver cells that makes them produce a chemical sponge for harmful cholesterol.