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.
My question relates to travel times of the Galileo and Cassini space probes to the planets. My question is why did Galileo take so long (6 years) to reach Jupiter? It was launched in 1989 and reached Jupiter in 1995. Cassini was launched in 1997 and reached Saturn in 2004. That's 7 years travel time to Saturn -- why so long? Pioneer 10/11 only took a couple of years to reach Saturn as did Voyager 1/2- launched in 1977 reaching Saturn in 1980/81. So why did it take Cassini twice as long to get there?