Can't remember the last time I fasted for even 2 days... But with intermittent and 24-hour fasting the only way I managed to do it was using caffeine. Specifically caffeine pills and home iced green tea, less than 200mg/day. They work wonders and shred through fat in combination with low-/med-intensity aerobic exercise and fasting, and keep brain active. Best done a few times a week only... Without caffeine I can't fast more than about 16 hours. Specifically I seem to require carbs or go braindead, high protein and any kind of fats (including short-chain fats) don't cut it, and I think too much fasting with caffeine worsened my problems in the medium-term, so I stopped intermittent fasting entirely some time ago. Probably worsened adrenals, thyroid, or god knows what now, and definitely impacts sleep. But of course caffeine had a role in that. It trimmed the fat at least. I have no doubt fasting can help the immune system but as they point out in the perfecthealthdiet carbs also have a role in immune response. So it must depend highly on the person and type of infection, in terms of whether it will help (or do more harm than good).