Immunity has a major factor in whether you get the flu. That is what flu shots are for. If there is depression, grief, fatigue, poor sleep immunity is reduced.
If there is humor, good sleep, absence of debilitating illness or medications that reduce overall immunity, then immunity / resistance is lowered.

The answer seems obvious at the surface level and most likely in extreme immune-deficiency states, it's a yes. However, for the typical healthy person, the answer is not known as a 1918 study of the influenza pandemic showed by inoculating the noses of healthy volunteers with the flu virus and none of them developed flu while other randomly associated people did. The reason some people developed the flu and others in the same situation did not develop the flu was not discovered.

The immune system is a complex integrated system (Immunology & Neurology) including immunocompetent cells and soluble factors that serve specialized roles. Immune responses can be highly specific (adaptative immunity) or nonspecific (innate immunity). The regulatory factors (cytokines and growth factors) influence immune cell development, activation, proliferation, differentiation, and overall immune function

Positive Aspects of Immune System:
Immunity include defenses against infections and cancers. Fever is an elevation of body temperature mediated by the hypothalamus in response to exogenous pyrogens and pyrogenic cytokines. Fever is also manifestation to other health conditions related to inflammation.

Negative Aspects of Immune Systems:
The negative aspects, including different hypersensitivities that can lead to pathophysiological conditions and autoimmune diseases that includes, rheumatic diseases, Liver Disease, osteoarthritis, inflammatory bowel disease, and psoriasis and many more. Autoimmune diseases are results of a array of conditions with a variety of presentations and the involvement of multiple organs. Although the sources & etiologies of many autoimmune conditions are unclear, it is believed that uncontrolled inflammatory immune response is the major cause of disease development and progression. The association between autoimmunity and cancer is well established.

Therefore immunity or immune system is more than having flu or fever.

Nancy asked: How does flu shot help? Essentially any flu shot is given to wake up the immune system and produce chemical and cells that combat that specific virus. Similar to Polio and Smallpox and many others.
Scientists try to determine which of the thousands of different flu bugs are coming next year and sometimes they guess right, in which case the immunity is excellent, and sometimes they miss it widely as in 2017 and today. The reason is that the virus can mutate and be more deadly or be unrecognized by our defenses.

Why did the trained athlete die of the flu? One theory is that from childhood he was raised in very clean environment, was given antibiotics with the slightest sniffle, and his body did not go through the standard colds and sore throats the most kids go through in order to build up their immune systems.

What is good for the common cold? Netflix! Yes, you watch a funny movie, drink lots of green tea with lemon and honey. Most of my patients report it works.

I'm Dr. Joanny Liu. I was invited to contribute to this question. My take on immunity is very different.

My background is Classical Chinese Medicine. I do not look at dis-ease as the root cause of disease, or in this case, the lack of immunity. It is very important to look at people not as merely a body but at what makes one human. It is the emotional state and having feelings.

You see, whenever we have a thought, we take action due to that thought. Now sometimes that action may be hesitation, but the origin of anything we do or not do, is due to the fact that we have a thought about it.

Therefore the body follows/obeys the mind.

The mind/body connection is not hard to explain. We already know that when we are angry, our blood pressure goes up, for instance. It may also interfere with our digestion. So it's not a good idea for some people to argue during meal times.

When we are afraid, we may sweat. We may freeze. We may run. But we are always driven to do something. It's not just instinct with human beings.

So the immune system is very much affected by our experience and our emotional response which comes from a thought. Haven't you ever witnessed or even had it happen to yourself, when you have a highly charged argument with someone, that you "catch" a cold within the next few hours or the next day?

Now if we are in control of our thinking, we will not allow temporary circumstances to invade our thinking. If we are trained well enough, we will take changes in stride.

And make adjustments calmly to those changes, understanding that we cannot control everything (hardly anything really) that happens to us.

The flu is an effect. The cause is over wrought emotion.

If you believe in a flu season, then sure enough you'll be participating with all the other believers.

I choose to be healthy, no matter what is going on around me.

Every now and then I do "catch" a cold, as I did as Christmas Day last year drew to a close. I was in bed all day Boxing day. This is a VERY rare event for me.

I realized later that there was much confusion for me at that time. Colds and flus do have that element of mental confusion and lack of clear thinking. You're listless and tired too. Well, it begins with feeling confused and being unable to make a decision.

Every time I work with clients, there is always a strong negative emotional reaction to something that happened to them, many times during their childhood, if it's a chronic problem that's lasted decades. When I help them let go of the (emotional) pain, the physical issues disappear.

I help them see how strong they really are. I don't tell them that they are. No, I give them exercises to show them themselves how strong they always were. Their self-confidence soars. They dream again and strive towards new goals. It is so awesome to see people transform from every day life to be more of who they really are.

The results can be astounding.

Every good doctor is also a very good psychologist and I don't mean old talk-therapy or cognitive behaviourial therapy. No, we must dig deeper to find the root cause of disease, anything that compromises the disease. And it is always mental/emotional.

Life is no longer a mystery.

Of course, immunity is part of the equation. The other part of the equation is how virulent the virus is and how much of it got in you (inoculum). Nutritionists emphasize bolstering immunity, while Germ Theory proponents (old school physicians) emphasize virulence and inoculation. Fortunately, the nutrition is gaining ground.

Take zinc for example. Here’s a nutrient shown repeatedly to reduce the duration of cold, perhaps by as much as 50%, according to WebMD, and may reduce the number of upper respiratory infections in children.  Zinc is involved in wound healing, which impacts cold and flu duration. Zinc is also involved in immune surveillance. That’s why it’s used as a preventive, along with vitamin C and echinacea, to bolster defenses against infection. Indeed, several anti-inflamatory and antioxidant nutrients from food and supplements can impact oxidative stress and healing. Vitamin D is also anti-inflammatory, promoting the formation of antimicrobial peptides to bolster innate defenses and fight off infection. Vitamin A promotes acquired immunity, directing antibodies/killer cells to specific infections.  Elderberry and oregano oil have also been shown to help prevent/reduce colds and flu. N-acetyl-cysteine (NAC) is a component of glutathione, the master antioxidant in every cell in your body. NAC has been shown to heal wounds, relieve congestion and prevent the flu. Probably the most important thing you can do to prevent flu, besides practicing avoidance and frequent hand washing, is to stay hydrated. This keeps your mucus membranes functioning well to flush out invading organisms. Zinc and vitamin A deficiency is associated with a lower production of IgA, the immune glycoprotein in your mucus membranes that prevents infection.

Some of these nutrients are not easy to come by. You don’t have to be from a third-world country to be hurting for good nutrition, especially if your diet has been Americanized. Vegans are often deficient in zinc, and may not be able to convert carotenoids (beta-carotene, lycopene) from plant sources into vitamin A (I am one of them.). Most people don’t get enough vitamin D, especially this time of year. There are numerous antioxidants and anti-inflammatory nutrients we get from wholesome foods that the typical American is not consuming. Eating or drinking too much sugar depresses immune cells that attack bacteria and viruses. This effect can last for hours. Eating more fruits and vegetables, rich in vitamins C and E, plus beta-carotene and zinc, will bolster immunity. But get some real vitamin A from animal sources to be sure.

This is what optimal immunity is all about: getting the nutrients to make it work properly. Indeed, there are many ways to bolster immunity to fight off all types of infection. To learn more, consult a wellness professional, such as a naturopath, nutritionist, integrative physician or a person qualified in this area of research and practice.

Your own level of immunity is based on many factors but in my experience, nutrition plays a huge role in how healthy we are coupled with an active lifestyle and a good balance of emotional & environmental factors as well.

Healthy eating including lots of fruits and vegetables, proteins and moderate sweets helps to keep the body in balance. I'm also a firm believer of supplements like Lysine (immune booster), Vitamins B & C and Omega 3's also help to fight off virus. I have been following this regimen for over 15 years and I can't remember the last time I had the flu... and I've never had the flu shot.

All of these suggestions are important to read , understand and use in our lives to make sure we all remain healthy, by using over the counter medications to vitamins to getting routine vaccinations. Any of these will help booster our immune system or lessen the symptoms once a disease infects our body's.