How long does a virus stay active in the blood?

The answer to your question is: it depends on the kind of virus you're talking about.

Whenever any virus invades our bodies, our immune system starts to attack it. Most of the time, our immune system is able to completely get rid of the virus. The immune system also develops a "memory" of the virus. So the next time the same virus invades our body, the immune system attack is even more effective.

So for most viruses, the answer to your question is: not long. Within days or weeks, most viruses are gone from our blood. And from everywhere else in our bodies.

But some viruses can "hide" inside certain cells in our bodies, and avoid being totally removed by the immune system. Some viruses can do this for a long time. Some can even cause a permanent, life-long infection.

Fortunately, many viruses that can cause a life-long infection stay "asleep" inside our cells — not making copies of themselves, and not causing any illnesses.

An example is the Epstein-Barr virus. Over 90% of adults in the U.S. have been infected with this virus, usually before young adulthood. Some people get infectious mononucleosis ("mono") when they first get infected with this virus. Others just get the symptoms of the common cold. And others don't get any symptoms at all. But regardless of the symptoms, the virus stays in certain white blood cells, cells in the throat, and possibly elsewhere — for the rest of our lives. Now and then, it can cause serious diseases. But almost always, it never causes any trouble.