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.
It launched on January 19, 2006 and passed the orbit of Saturn on June 8, 2008. It completed this journey in just 2 years and 4 months. Just in case you were wondering, here's how long it takes to fly to Mars, and how long it takes to get to the Moon.