Woodmansey et al. (2004) found a significant decrease of acetate and butyrate in healthy elderly compared to antibiotic-treated elderly patients. This supports the use of probiotics in the elderly to enhance natural killer cell activity and quantity (Gui et al., 2020) and decrease IL-6, IL-8, and C-reactive protein (Costabile et al., 2017). A randomized trial found that COVID-19 patients who took probiotics had shorter duration of symptoms, lower viral loads, and a higher rate of complete remission compared to COVID-19 patients in the placebo group (Gutiérrez-Castrellón et al., 2022). Here, IL6 is linked to COVID-19.