There is, believe it or not, a scientific theory regarding this: panspermia.

According to the panspermia hypothesis, microbes "hitch" a lift on bodies leaving a planet. They then travel through space and eventually land somewhere else.
Panspermia involves three difficult phases of travel:

• Launch (as well as the impact event)


• Travel in the harsh environment of space


• Atmospheric entry and landing, with high temperatures

It's difficult to survive these. However, materials related to organic matter have been found that might be evidence of panspermia; see Bell et al. (2015) for one example.

Panspermia is traditionally applied to bacteria known as extremophiles, which are accustomed to living in harsh environments. Space contains temperatures ranging from hundreds of degrees Fahrenheit to hundreds of degrees below zero, and any organism would have to survive those. This may be possible - bacteria have survived in conditions like there - but difficult.

Viruses pose an additional challenge because they cannot survive long without a host organism on which to live. This means they would have to be carried along by an extremophilic bacteria. This also means that they might not be a huge harm to creatures on Earth. Viruses only attack certain bacteria because of proteins on their surfaces; extraterrestrial biology most likely differs from that on Earth, and these viruses might not be equipped to harm terrestrial organisms.