SLEEP AND CIRCADIAN CLOCK
We’re all aware that our bodily functions benefit from sleep. In addition, sleep helps your body fight infection, since “sleep has the potential to enhance the efficiency of T cells (which are a type of white blood cell that are critical to the body's immune response)" by improving the ability of T cells "to attach to their target cells and, in the case of a virally infected cell, kill it.” 
Optimizing the quality and duration of sleep can sometimes be the best medicine:
7-9 hours of high quality sleep per night is important.  This means forgoing tendencies to stay up and going to bed at a reasonable time. Working backwards from a 6 a.m. waking up time, the ideal time for bed works out to be 10 p.m.
To improve sleep quality we try to optimize the duration of Stage 3 (deep) sleep, "this is the stage when the body repairs and regrows its tissues, builds bone and muscle, and strengthens the immune system.",  Remember - there is always scope for improvement in sleep quality!
"Your body has a natural clock, called a “circadian clock,” that helps you regulate your sleep. The word “circadian” refers to rhythmic biological cycles that repeat every 24 hours. Your circadian clock is strongly influenced by light and dark in the environment."  Optimizing your circadian rhythm can help improve your sleep quality and immune status. 
SUNLIGHT & VITAMIN D
There is strong evidence that vitamin D plays a crucial role in a healthy immune response since it “protects against acute respiratory infection overall.” 
“Through several mechanisms, vitamin D can reduce risk of infections. Those mechanisms include inducing cathelicidins and defensins that can lower viral replication rates and reducing concentrations of pro-inflammatory cytokines that produce the inflammation that injures the lining of the lungs.”  Also, “Vitamin D supports induction of antimicrobial peptides in response to both viral and bacterial stimuli. Metabolites of vitamin D have been shown to induce other innate antimicrobial effector mechanisms, including induction of autophagy and synthesis of reactive nitrogen intermediates and reactive oxygen intermediates.” 
There are two methods of obtaining vitamin D in the body:
Vitamin D3 supplementation
Obtaining vitamin D from sunlight is the better way to go since “all-cause mortality was inversely related to sun exposure habits. Mortality rate amongst avoiders of sun exposure was approximately two-fold higher compared with the highest sun exposure group, resulting in excess mortality.”  However, many of us can’t obtain enough vitamin D from the sun. So, adequate supplementation is important. “The goal should be to raise [blood vitamin D] concentrations above 40–60 ng/mL.” 
Vitamin K2 deficiency produces symptoms of vitamin D toxicity, which includes inappropriate calcification of soft tissues that can lead to atherosclerosis.” It is therefore suggested to take “100 mcg of vitamin K2 for every 1,000 IUs of vitamin D.” 
SUGAR AND ALCOHOL
Sugar suppresses immunity by “significantly decreasing the capacity of neutrophils [type of white blood cell] to engulf pathogens”  Therefore try to avoid sugar and reduce carbohydrates. Your sweet tooth can be satisfied with fruit!
“Alcohol negatively impacts your immune system with even moderate amounts of alcohol influencing immune responses.“  Therefore its best to avoid alcohol as much as possible.
TIME-RESTRICTED EATING & INTERMITTENT FASTING
Time-restricted eating means constricting your eating window during the day. One can do this by skipping dinner, skipping breakfast, or reducing the time between your meals. Typically the eating window ranges between 6–10 hours. Fasting for 24 hours, one or two days a week, is also beneficial because “periodic cycles of fasting have systemic anti-inflammatory effects and increase progenitor stem cells.” 
“Cycles of prolonged fasting not only protect against immune system damage but also induce immune system regeneration, shifting stem cells from a dormant state to a state of self-renewal.”  “Prolonged fasting cycles — periods of no food for two to four days at a time over the course of six months — kill older and damaged immune cells and generate new ones.” 
SIT LESS & MOVE MORE
We all know the amazing benefits of vigorous exercise. However, let's not use exercise as an excuse to be sedentary. “Prolonged sedentary time was independently associated with deleterious health outcomes regardless of physical activity”.  “Both the total volume of sedentary time and its accrual in prolonged, uninterrupted bouts are associated with all-cause mortality”. 
This implies that one should aim to reduce the total time spent sitting, as well as increase the frequency of movement breaks in a day. Good alternatives to sitting are standing desks that encourage active movement. If you are sitting, try to get up every 30 minutes for a little movement like walking to get yourself a cup of water/tea/coffee.
ZINC, SELENIUM, & VITAMIN C
Zinc has been shown to be very effective in combating viral infection.“ A combination of Zn2+ and PT at low concentrations (2 µM) inhibits the replication of SARS-coronavirus (SARS-CoV) and equine arteritis virus (EAV) in cell culture.”  Great food sources of zinc include oysters & seafood, while the recommended form of zinc supplements are zinc glycinate or picolinate.
Selenium is also shown to be effective in reducing the effects of viral and bacterial infections, let alone the fact that a deficiency in selenium is tied to an increase in all-cause mortality.  A great food source is 3-5 brazil nuts per day, while the recommended form of supplemental selenium is selenomethionine.
Vitamin C is well known to be helpful in immunity, let alone the multitude of systemic benefits that it has. “It stimulates the production of white blood cells, especially neutrophils, lymphocytes, and phagocytes, and also promotes the cells normal functions such as their ability to detect, move towards, and engulf pathogens."  The highest food source of vitamin C is the Indian gooseberry (also known as amla) which provides 20 times more vitamin C than an orange!
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