The following factors typically affect your immunity: 

‍Age

‍Your immune system weakens as you advance in age, making it less effective. As you age, your body produces fewer white blood cells, making it harder to fight off infection. The skin and other physical barriers also become less effective at protecting you from germs. That's why older adults, especially seniors are more susceptible to illnesses like the flu, pneumonia, and shingles. 

Sleep & Rest‍

When you sleep, your body produces cytokines – proteins that help control inflammation. Additionally, enough sleep helps your body rest and repair the damage caused by inflammation. Sleep deprivation can increase the chances of you suffering from depression, obesity, diabetes, and heart disease, further weakening the immune system. The Centers for Disease Control (CDC) recommends the following sleep amounts:

• Infants - 12-16 hours
• Toddlers and preschoolers – 10-14 hours
• Preteens and teens – 8-12 hours
• Adults – 7-9 hours

Exercise‍

Regular exercise helps to increase the production and circulation of white blood cells, antibodies, and cytokines. Additionally, exercise helps to reduce inflammation and produce endorphins – feel-good hormones – that reduce stress and improve your mood.

‍Stress

Chronic stress increases the production of stress hormones such as cortisol and suppresses the production of infection-fighting components, such as white blood cells. Additionally, experts state that stress can damage the cells in the gut by altering your gastrointestinal movement and secretions, enhancing gut sensations, and negatively affecting your intestinal microbiota. 

When you're highly stressed, you're at a higher risk of infections, autoimmune diseases, and cancer. 

Environmental Factors

Your immunity can weaken when exposed to certain environmental factors, such as air pollution, cigarette and secondhand smoke, toxins, and climate change. Certain chemicals, such as pesticides, cause inflammation and damage your respiratory system, making it harder for your body to fight infections.

Diet and Nutrition

Nutrients are critical for all bodily functions, including the immune system. Specifically, nutrients help the immune system in several ways, including:

• Maintain skin and mucus membranes as a barrier to infection  
• Regulate enzymes and proteins that protect cells from virus and bacteria
• Produce antibodies and white blood cells that respond to infection
• Support growth and activity of immune cells
• Support immune system signaling and response

A well-balanced diet that includes fruits, vegetables, whole grains, good protein, and healthy fats should provide most of the nutrients you need for good health and a well-functioning immune system. The most supportive nutrients for the immune system include:

• Vitamin C – can be found in leafy green vegetables like spinach and kale, bell peppers, broccoli, strawberries, cantaloupe, baked potatoes, and tomatoes
• Vitamin D3 – can be found in fatty fish (salmon/tuna), cheese, egg yolk, and mushrooms. Your skin can also produce vitamin D3 when exposed to sunlight
• Vitamin E – can be found in vegetable oils, nuts and seeds, and green vegetables
• Selenium – can be found in seafood, meat and poultry, eggs, dairy products, breads, cereals, and grain products
• Zinc – can be found in oysters, red meat, poultry, seafood, beans, nuts, dairy products, and whole grains
• Omega-3 Fatty Acids – can be found in fish, algae, and flaxseed