Well, clean air helps. Avoiding pollution and second-hand smoke is important. Suburban and country air tends to be better than city air. Outdoor air tends to be better than indoor air. If allergens and large contaminants are a problem, a HEPA filter can be useful.
Next, a well-rounded diet and exercise help. People with respiratory disease cannot tolerate abnormal weights. If they are overweight, then they have to lift a lot of fat on the chest wall with every breath. If they are underweight, then they lack some of the muscle mass needed for their increased work of breathing. With most overweight people, the cravings are usually for “denial diets.” That is, if you exclude certain foods, then your weight will come down. The problem with denial is that soon or later you have to admit that you actually enjoy and crave those foods and it leads to binge eating. It is better to make good food choices and watch the portion size of everything that you eat. If there is a big difference between the serving size on the packaging, and the portion size on your plate, you know what the problem is. The other problem is that people want to “finish their plate” because of the “starving children.” If while you are eating, you discover that you are satisfied and no longer hungry, then stop eating and put the food away or throw it out. The best way to help starving children is to write a check to your favorite charity. It is also true that you should eat some fruits and some vegetables every day. It is much better to get your vitamins from food then from a bottle. However, there is some evidence that people who live in a selenium poor area may benefit from selenium supplementation to prevent lung cancer.
Regarding underweight people, it is not easy to “bulk up” when you are over 60 years old, have a poor appetite, and are short of breath. There are many nutritional drinks and high calorie shakes that can help. You can even make your own in a blender with pasteurized eggs, ice cream, milk or yogurt. If the weight loss becomes life-threatening, then some people may benefit from what I candidly refer to as the “illicit athletic” approach, which is anabolic steroids and appetite stimulants. In the most extreme cases, a feeding tube with nighttime feedings (in addition to daytime meals), may help raise weight.
One last thought is regarding being in public. Often in public places, you will encounter someone who is sick and either sneezing or coughing. Or you yourself, may be sick and coughing. It is best for the sneezing or coughing person to cover their nose and mouth when coughing. Many organisms can be transferred from person-to-person via aerosols or droplets. Keeping a distance from such people is beneficial if they are not covering their mouths. Also, remember that these droplets may be present on objects which you touch. Try to wash your hands with soap and water after coming into contact with commonly touched objects (telephones, doorknobs, et cetera). Alcohol gel is a second choice when soap and water are not available. Especially be sure to wash your hands before eating or touching your face.