We've always had marginal/debatable illness. The existence of Chronic Lime Disease is debated, similar to Fibromyalgia, or perhaps Chronic Fatigue. Havana Syndrome is a more recent example. It's possible that these are actualy manifestations of psychological conditions like depression/anxiety, but people have a hard time accepting that. Doctors debate this, and I'm not a doctor. Who knows, maybe these are lingering conditions brought on by flus or other viruses.

You never heard of post-viral chronic fatigue?

Long flu is a thing [0] [1]. People were talking about that for years after the 1918 flu.

There's some evidence that a prior flu outbreak, the Russian Flu around 1890, may have been an earlier coronavirus pandemic [2], but nobody could differentiate them at the time.

EDIT: The parent's question seems reasonable. Covid is not unique amongst viruses in the damage it causes. Besides long flu there's things like myocarditis [3] from the flu that may have been neglected in the past, also.

[0] https://www.sciencefocus.com/news/long-flu/

[1] https://time.com/5915616/long-flu-1918-pandemic/

[2] https://pubmed.ncbi.nlm.nih.gov/34254725/

[3] https://pubmed.ncbi.nlm.nih.gov/33437555/