Does green tea help against infections?
For flu there is reasonable evidence that green tea catechins help; for most other infections the evidence is still limited to laboratory research, so use it as a supplement to conventional prevention, not as a replacement for it.
Green tea has the most proven benefit against influenza. A meta-analysis of five studies involving 884 participants showed that people who took green tea catechins got flu roughly one third less often than people who received a placebo. Three population studies with 2,223 participants confirmed that picture. If you want to try it for flu prevention: drink green tea daily, several cups, or take a catechin supplement, throughout the flu season. Check the label for 'green tea catechins' or 'GTC'. Whether you gargle or swallow made no difference in the analyses, so that does not need to factor into your choice. An exact amount in milligrams has not been established; the studies varied in this regard.
For other viruses, such as hepatitis, herpes and HIV, there is only laboratory research. The active component, EGCG (the main substance in green tea), inhibits these viruses in cell culture studies, but whether that also works in people has not been investigated. For COVID-19, the evidence so far consists of theories and computer models, not trials in people. This is too poorly studied to recommend, including as a supplement with 'EGCG' or 'green tea extract' on the label.
For bacterial infections the picture is mixed. EGCG inhibits bacteria in the laboratory, but the concentrations required are ten to one hundred times higher than those needed for virus inhibition. Whether those concentrations can be reached by drinking tea has not been established. One small study in 64 patients with a skin infection, not compared against a placebo, showed that tea ointment (applied externally, 'tea ointment' on the label) had a healing rate of 81%, comparable to standard antibiotic ointment. That is too little evidence for a recommendation; always discuss a skin infection with your doctor.
Drinking green tea is safe in normal amounts. As a supplement to existing preventive measures, such as annual flu vaccination, drinking green tea daily is not an unreasonable choice. It is not a replacement for conventional treatment of an infection. For the common cold there are only indications from population studies; clinical evidence is lacking, and for that reason it is not recommended as a targeted measure either.
Evidence for influenza prevention comes from a meta-analysis (PMID 34209247). Antiviral laboratory data come from reviews (PMID 23072320, 34209485) and cell-culture studies (PMID 11684313, 32868200). Antibacterial laboratory evidence from a review (PMID 23072320). Impetigo clinical study: PMID 11138536. Common cold: review (PMID 30037024). COVID-19: reviews (PMID 34209485, 38291871).
Does sauna bathing help against infections?
There are preliminary indications that regular sauna bathing may have a beneficial effect on your immune system, but the evidence is too thin for firm conclusions. If you have joint complaints or an unusual skin condition, discuss it with your doctor before making it a regular habit.
Can too little sleep make you more susceptible to infections?
Too little sleep demonstrably makes your immune system more vulnerable, including to infections and vaccinations. Make sure you get sufficient sleep, even though the exact numbers and thresholds are not yet precisely known.
Is zinc useful for my immune system?
Zinc supports your immune system, especially when you have a deficiency, and can help symptoms of a respiratory infection clear up more quickly. It does not make you less likely to fall ill, but recovery is on average faster.
Does regular exercise help against that creeping chronic inflammation?
Regular, moderate exercise demonstrably lowers inflammatory markers such as CRP and IL-6; training too intensely can reverse that effect, so moderate exercise three to five times per week is the best-supported choice.
Which supplements genuinely support your immune system?
Some supplements genuinely support your immune system, but the effect is greatest when you have a deficiency: so first check whether your vitamin D or zinc levels are adequate, and supplement in a targeted way.
Can you strengthen your immune system by eating more fibre?
Eating more fibre has a clear biological mechanism, but in healthy people it does not automatically lead to a measurably stronger immune system. Fermented foods appear to work more reliably for your gut flora; probiotic supplements are even risky in the context of immunotherapy.