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.
Using a chemical propulsion system, it took the Apollo astronauts about 2.5 days to get to the moon, traveling at speeds faster than 39,000 km/h (more than 24,000 mph). The space shuttle uses a chemical propulsion system based upon both liquid and solid propellants.