Three o’clock on a weekday afternoon, the sun filtered through the half-open blinds. I knew it was a nice day outside but couldn’t bring myself to move. Two weeks before, I had developed shaking chills, vomiting and diarrhea. In the emergency room I learned I had norovirus, a gastrointestinal virus that people get on ships and in nursing homes.

Now, though most of my physical symptoms had resolved, a kind of inertia prevented me from doing anything. I looked hopelessly at the pile of papers and mail that sat on the dining room table, exactly where I left them the night I got sick. I didn’t want to be with anyone. All I wanted was to lie on my bed, click TV channels, doze and feel sorry for myself.

Was this what post-viral depression felt like? I knew what sickness behavior was – that desire to sleep, not move, the loss of appetites of all kinds, loss of interest in other people and the outside world. This turning in on oneself is protective to the sick individual and also to the species; it conserves energy for recovery and isolates the ill animal from the herd. Those feelings felt normal while I was physically ill, but this was different. It seemed that my mood symptoms were lagging behind my physical symptoms.

Immune molecules can bring about these symptoms, whether the immune system is activated by infectious agents like viruses or, in the case of lupus or autoimmune forms of arthritis, by inflammation itself. Immune cells fighting off the invader trigger signals to the brain that inflammation or infection is occurring in the rest of the body. This sends the brain into sickness-behavior mode, which lasts about 48 hours until the infection is cleared or inflammation resolves.

In some cases, a second wave of sickness-like behaviors can occur that mimic the symptoms of depression: a sense of sadness, hopelessness, inertia, loss of appetite, loss of interest in sex, isolation from friends and family. In short, all those things that I was feeling.