All the below are 'self help' immunity pointers and do not replace professional medical advice
Sleep - very underrated but it plays a huge part in susceptibility to illness. Get as much sleep as you can for your body system to repair
Nutrition & water - Research the immunity value of many fresh fruits and vegetables such as broccoli, kale, greens, lemons, apples, bananas, potatoes. Eat more of them and drink lots of water. Eat a little bit of protein and cut out all the processed junk that is widely available (eg crisps, cakes, candy, chemical soups, sugar laden products)
Exercise - Exercise is hugely beneficial to keeping a good oxygenated blood flow to the body organs, muscles and lymphatic system. Walking is good for your health. Too much exercise can cause the immune system to break down - this is extreme exercise where the body's recovery system cannot repair old cell damage.
Stress - Monitor when you succumb to illness and get to know your personal patterns. Often it is when we are emotionally upset that we get every ailment under the sun. Also when we are tired, run down and out of sorts it becomes very difficult to climb back out of this state and you are more susceptible to illness.
Kids - If you have any school age or nursery age kids they are pure lurgy factories. They go and collect every known cough cold and bug known to man. Kids rub shoulders with everyone and then bring their viral and bacterial collections home to Mum and Dad.
If you are still worried or concerned about your health then go back to your GP or health provider and talk to them to see if there is something they have missed or anything else that might be causing you to be unwell.