> Is my friend a virus for recommending me their favorite book?
In that case the book would be a virus, and your friend a host.
But it generalizes way wider to ideas. Religions are viruses too for example. With very strong viral mechanisms sometimes. Some religions are so much entrenched so that they have become part of the fabric for entire societies or countries, morphing their culture; the same way that sometimes viral RNA can become part of the DNA of the host.
In that case the book would be a virus, and your friend a host.
But it generalizes way wider to ideas. Religions are viruses too for example. With very strong viral mechanisms sometimes. Some religions are so much entrenched so that they have become part of the fabric for entire societies or countries, morphing their culture; the same way that sometimes viral RNA can become part of the DNA of the host.