Why might people be taking the idea of sending viruses into space seriously?

Answer 1

Ask the Native American population about the smallpox virus. We would not want to bring along diseases (viral, fungal, bacterial, etc) that might be able to attack native life should we encounter it.

Also, it complicates the search for life if we introduce contamination--i.e. we send a probe to Mars, return some samples with complex organic molecules, and cannot determine whether it is life from Mars or contamination.