Prevention depends on the cause. For example, weakness due to lack of exercise can be prevented with regular exercise. Weakness due to alcohol or diabetic neuropathy can be prevented by not drinking alcohol or controlling diabetes with a proper diet and medications. Many cases of weakness cannot be prevented.
Weakness, when severe, may prevent people from doing normal activities. Most of the serious long-term effects are due to the cause. For example, people who have had a stroke may become paralyzed for life. People who are weak from an infection may have no long-term effects after treatment. People with cancer may die if treatment fails.
Weakness is not contagious and poses no risk to others. But if the cause of weakness is an infection, the infection may be contagious.