If you want to fight back to the illnesses, the key factor is to improve your immunity which would help your body fight back to illnesses.

Here are some tips to improve your immune system.
1. GET ENOUGH SLEEP
Getting adequate rest may strengthen your natural immunity. Also, you may sleep more when sick to allow your immune system to better fight the illness.
Adults should aim to get more than 6 hours of sleep (the best is more than 7 hours) each night, while teens need 8 hours or more and younger children and infants up to 14 hours.
SUMMARY
AS I HEARD ADULTS SHOULD GET 6 HOURS OF SLEEP, ACTUALLY WE SHOULD SLEEP 7 HOURS OR MORE WHICH WOULD DECREASE THE RISK OF GETTING SICK.
2. INCLUDING FRUITS AND VEGETABLES
The fiber in plant foods feeds your gut microbiome, or the community of healthy bacteria in your gut. A robust gut microbiome can improve your immunity and help keep harmful pathogens from entering your body via your digestive tract.
Furthermore, fruits and vegetables are rich in nutrients like vitamin C, which may reduce the duration of the common cold
SUMMARY
EATING FOOD WHICH CONTAINS FIBER AND VITAMIN C SHOULD HELP IN THE CASE FOR IMPROVING IMMUNITY.
3. DECREASE SUGAR
Curbing your sugar intake can decrease inflammation and aid weight loss, thus reducing your risk of chronic health conditions like type 2 diabetes and heart disease.
Given that obesity, type 2 diabetes, and heart disease can all weaken your immune system, limiting added sugars is an important part of an immune-boosting diet.
You should strive to limit your sugar intake to less than 5% of your daily calories. This equals about 2 tablespoons (50 grams) of sugar for someone on a 4,000-calorie diet.
SUMMARY
ACTUALLY ABSORBING MORE SUGAR THAN YOUR BODY NEEDS WOULD LEAD TO 2 DIABETES AND HEART DISEASES. LOWERING YOUR SUGAR INTAKE WILL DECREASE YOUR CHANCE OF THESE CONDITIONS.