There are around 200 or more types and sub-types of virus that contribute. Further, viral DNA changes frequently. This makes preventing and treating RTIs quite difficult and helps explain why we tend to get many RTIs throughout our lives. Although we tend to think of a few days off work/school with a warm blanket, RTIs can be very serious, especially in those with a weakened immune system such as the elderly or those with certain medical conditions.
There is some evidence that low-fat, high-carbohydrate diets enhance the immune system. 
RTIs are usually spread through contact with others who are infected, for example from exposure to coughs, sneezes, and even breath. With this in mind, RTIs spread rapidly when people are in confined spaces: think of schools, large office blocks, public transportation, etc. Contaminated surfaces such as hands are major sources of transmission. For example, a person touching their nose, and then shaking your hand or touching a surface that you then touch with your own hand.
Therefore, one of the most important aspects to preventing RTIs is ensuring good hygiene practices, particularly regular hand washing. Of course, we don’t all wash our hands repeatedly everyday. Even if we practice good personal hygiene, it is unlikely that every individual we come into close proximity with does! This makes it almost impossible to avoid germs from others. Fortunately, there are other things that can influence our immune system—including a healthy diet—all of which can decrease RTI risk even when germs are all around us.
7 Factors Associated With Decreased Immune Function
1. Excess alcohol
2. Poor sleep 
In one study of over 22,000 Americans, a short sleep duration was associated with increased RTI risk of up to 82%. In another study, healthy volunteers were exposed to ‘nasal drops containing a rhinovirus’ (in other words mucous infected with the cold virus). Those who regularly sleep less than 7 hours per night were 200% more likely to develop a cold than those with 8 hours of sleep or more.
3. Intense exercise
4. Obesity 
increases both the risk and severity of RTI. Being overweight is associated with decreased immune response to influenza virus.
5. Excess dietary fat
6. Smoking
7. Psychological stress 
has been shown to weaken the system. 
7 Factors Associated With Superior Immune Function
1. Low-fat, high-carb diets 
2. Micronutrients, including vitamins (like vitamin C)
3. Vitamin D 
4. Minerals and trace elements
5. Phytonutrients
“Phyto” means plant, so phytonutrients are non-essential but highly beneficial nutrients found only in plant foods. 
6. Fiber – In the past, dietary fiber was thought to be indigestible and only beneficial in promoting gut health. We now know that fiber can be at least partially digested by bacteria living in our guts. 
7. Moderate and regular exercise are important for overall health, including the immune system.
Nutrition and Immune Function
There is evidence regarding nutrition and RTIs specifically, and some of the available evidence is very interesting.
A study from the late 1980s reported that the total number of white blood cells did not differ between meat eaters and those following a plant-based diet. However, one very important type of immune cell (called natural killer cells) were more active in the plant-based group. This suggests increased protection against not only RTI, but also cancer. In fact, an earlier study from Australia reported that RTIs were lower in those eating a plant-based diet compared to meat eaters at every age group. Interestingly, RTI rates were higher in vegetarians than in those eating a high fruit and vegetable diet. This study suggests that eating a plant-based diet can be beneficial, but actually consuming fruits and vegetables as opposed to other plant-based foods may be optimal.
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