Viruses, in certain conditions, can benefit health and agriculture. One example relates to the development of the immune system and the possible replacement of beneficial bacteria in the gastrointestinal system. This is especially important following damage by antibiotic use.
Also within the immune system, viruses can arm the killer cells that fight tumor cells, as well as cells infected with pathogenic viruses.
In the mucus membranes, bacteriophage—the viruses that infect bacteria—may protect the body from invading bacteria.
In the plant world, symbiosis among viruses, bacteria, and fungi may be essential for the survival of particular species in challenging environments. An example is provided by grasses that grow in very high-temperature (over 100° F) areas, which have a symbiosis with a fungus and a virus infecting the fungus. Conversely, other viruses help with adaptation to cold. These cases suggest possible applications to expanding the ranges in which crops can be grown.
There also exist so-called "persistent viruses" infecting plants, meaning that they are passed down through generations—even for thousands of years—but not between different plants. White clover, for example, can contain the cryptic virus, which affects the formation of nitrogen-fixing nodules depending on the amount of nitrogen in the soil.