In this essay I will assume that UFO's are not intelligent visitors from other worlds. The next thing to do is to define life. How can we tell a living thing from something that doesn't live. Sometimes it's a little tricky. In fact, the line between living and non living is actually a very fuzzy one.
Viruses are very close to that line.
Away from a host cell, most viruses are crystalline solids composed of proteins. Within a cell, the material inside the virus uses the cell's chemistry to reproduce itself. So what we have to find is a working definition that will include all living things but not ones that are not living. That is actually very difficult. The one I will use for this essay is as follows.