That's a correlation, not a cause. That just means those two items might be related. For example, it could be that people with mental issues choose to eat fast food more often due to poor planning or decision making. It could be that poor people tend to both have more fast food and have more mental illnesses.

Put simply, I'd hesitate to use that study as evidence of fast food causing mental health issues.

The definition of "alive" is kinda fuzzy, but we're pretty sure that viruses aren't actually alive. It's probably easier to think of them like little machines that break into cells and convert them into virus factories. You can "kill" a virus by breaking it's outer shell, much like your phone would die if I snapped it's battery in half.