A primary reason is that viruses do not possess a cell membrane or metabolise on their own - characteristics of all living organisms. Examples of common human diseases caused by viruses include the common cold, the flu, chickenpox and cold sores.
In the past spacecraft have taken greatly different amounts of time to make it to Saturn. Pioneer 11 took six and a half years to arrive. Voyager 1 took three years and two months, Voyager 2 took four years, and the Cassini spacecraft took six years and nine months to arrive. The New Horizons spacecraft took a short two years and four months to arrive on the scene.
The first mission to stop on the moon with astronauts on it, Apollo 11, landed about four days after leaving the Earth surface. I believe I remember reading that the Saturn V rocket took three days to make the trip to the moon. The moon is about 220,000 kilometers from the Earth. You do the math.