When Shirley’s grandmother was hospitalized with the flu, its effects were swift and devastating, and far more severe than vomiting and headaches.

“It was shocking to see her after she was sent to a rehab facility. She had no facial expression at all,” said Shirley. “I was immediately frightened.”

Shirley talked to the nurses at the facility, who discouraged her from involving a psychiatrist or mental health professional, saying she was just “sick.” Shirley knew there was more to it than that, but deferred to whom she believed to be the experts.

“It is one of my deepest regrets that I did not push harder to get her some help,” Shirley said. “My grandmother was clearly depressed. She then slid into dementia, and was never again as mentally healthy as she had been before she got the flu.”

Is Shirley’s grandmother an extreme case or can the flu bring on mental health issues like depression for anybody? History shows the two have been closely linked for more than a century.

The Links Between Depression and the Flu

In 1892, Daniel Hack Tuke, a London physician, wrote a series of articles on “post-influenza mania and depression.” He observed that people who were admitted to the hospital with upper respiratory infections and flu had higher levels of depression than those admitted for other conditions.

Flu symptoms can include fever, body aches, cough, and nausea. Other symptoms of the flu look like the symptoms of depression–like lack of energy, loss of appetite, exhaustion, and difficulty sleeping and concentrating. Cycles of physical illness causing mental health issues, and mental health issues making physical health issues worse also exist.

But as is true with so much surrounding depression, there’s a lot we do not know.

Dr. Steven Schlozman, co-director of The MGH Clay Center for Young Healthy Minds, and a staff child and adolescent psychiatrist at Massachusetts General Hospital wrote a blog post on the psychological effects of the flu.

“ A psychiatrist worries about the flu for the same reasons that a pediatrician worries about depression. Feeling lousy from the flu makes the other things that make you feel lousy feel a whole lot lousier. By the same token, pre-existing depression, anxiety and other psychiatric syndromes can make it very difficult to recover readily from a bout of the flu.”

There’s another wrinkle: Both the flu and depression activate an immune response and cause neuroinflammation in your brain.

Schlozman notes that “Because psychiatric distress is associated with inflammation, and because the flu puts your body in a highly inflammatory state, it is likely that this inflammatory state worsens the body’s psyche.”

Psychology Today explains neuroinflammation.

“The symptoms of the flu are actually a result of our immune system trying to combat the virus. The symptoms of the flu are brought on by proteins, pro-inflammatory cytokines, that our bodies produce to fight the flu and other infections. When the immune system is under attack, the immune system generates an inflammatory response, which causes the release of cytokines.”

When our brain “hears” that our body is inflamed, we are programmed to feel down and tired with a sense of malaise, losing our appetite, tending to isolate ourselves socially, and generally wanting to crawl under a rock.

“Flu initiates a whole-body inflammatory reaction. The damage from flu continues even after one recovers from the acute flu illness. It’s why many people feel wiped out for two weeks or more after they’ve stopped coughing,” says William Schaffner, MD, in an article in Everyday Health. Schaffner is medical director of the National Foundation for Infectious Diseases (NFID)and a professor of preventive medicine and infectious diseases at the Vanderbilt University School of Medicine in Nashville, Tennessee.

Additionally, Dr. Schaffner indicated that flu in elderly people may precipitate a permanent decline in pre-flu function, increasing the risk of disability – just like what happened with Shirley’s grandmother.

And then there are more obvious reasons one might feel depressed after a bout with the flu—the accumulated effects of not eating, not leaving the house, and calling in sick to work or class for days or even weeks.

Keeping Depression at Bay

Although the Centers for Disease Control says that “the timing, severity, and length of the (flu) season varies from one season to another,” the 2017-2018 flu season was particularly bad, with a record number of flu deaths among children and the highest rate of flu-related hospitalizations among older adults, according to data released from the Centers for Disease Control and Prevention (CDC). An estimated 80,000 Americans died from the flu and flu-related complications last season, including 180 children. The news media is reporting the first death from the flu already this season.

If you or someone you love comes down with flu, and you suspect that there are also symptoms of depression, don’t wait – talk to your doctor. Depression symptoms can be treated at the same time you are treating the flu.

Shirley wishes she had listened to her instincts about her grandmother’s depression symptoms: “My Nana recovered from the flu and was physically healthy for a few more years, she just never got back to her pre-flu mental state.”

Once you start to feel physically better, your mental health should improve, too. And treating the depression can help you to start feeling better as well.