Some are obvious, some not so obvious.
Weakened immune systems are a result of many contributing factors.
Either because of a chronic health condition, lack of sleep, environment,dehydration, or nutrition.
Addressing the underlying cause will bring about a healthy immune system. There are incidence whereby the immune system attacks itself, as is the cause of auto immune diseases, such as MS, diabetes, psoriasis, and cancer.
Among the most helpful in these conditions has been vitamin D3.
Other conditions include chronic infection. Your immune system becomes weakened by these infections, and no longer can fight a common cold. Examples include chronic yeast infection, intestinal dysbiosis, CFS, fibromyalgia, and many others. These conditions have been linked to chronic infections with viral, bacterial, and fungus infection. By treating the underlying infection, boosting the immune system, and getting adequate sleep, you can force the condition into remission. During sleep our body rejuvenates itself, so getting adequate sleep is detrimental.
Environmental conditions also decrease immunity. Among these are air quality, mold, toxic cleaning agents, as well as a lack of fresh air. Your body is too busy fighting off these pathogens to keep you well.