AFAIU, catching any illness is likely to reduce your vitamin D levels. For one thing, the active metabolites are created by the liver and then the kidneys. http

AFAIU, catching any illness is likely to reduce your vitamin D levels. For one thing, the active metabolites are created by the liver and then the kidneys. https://www.cdc.gov/nutritionreport/99-02/pdf/nr_ch2b.pdf https://www.cdc.gov/nutritionreport/99-02/pdf/nr_ch2b.pdf Illness can depress this process. See, e.g. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6360762/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6360762/ Furthermore, if you're bed-ridden then you're not likely getting sunlight, and it only takes a day or two of no sunlight for your serum levels to drop significantly. Of course, none of this is to say that there's no reverse causative relationship as well, but people really need to take these correlation observations with a huge grain of salt. And also take note of extremely inconsistent results when controlled trials are attempted.