It constantly triggers the limbic system. Panicky stories spur the release of cascades of glucocorticoid (cortisol). This deregulates your immune system and inhibits the release of growth hormones. In other words, your body finds itself in a state of chronic stress. High glucocorticoid levels cause impaired digestion, lack of growth (cell, hair, bone), nervousness and susceptibility to infections. The other potential side-effects include fear, aggression, tunnel-vision and desensitisation.
I have developed a new reflex: in addition to the one where, any time I see a headline ending in a question mark I mentally shout "NO!" and don't read the article, I also find that any time I see a headline that promises me "Everything you need to know about..." the voice yells at me, "YOU DON'T NEED TO KNOW ANYTHING ABOUT THAT!" and I move on.
Thank you again, voice inside my head.