Prevention is related to the underlying cause. For example, avoiding alcohol can prevent nausea from drinking too much. Medications can help with the nausea caused by traveling. If the nausea is due to morning sickness during pregnancy, eating crackers and avoiding fatty foods can help. There are many other examples of prevention depending on the specific cause.
Nausea itself has no serious long-term effects, though it may be distressing and prevent sleep and other activities. The underlying cause may be very serious, however. For example, cancer is a rare cause of nausea that can lead to death. If vomiting occurs with the nausea, dehydration and salt imbalances may occur.
Nausea itself is not contagious and poses no risk to others. In some cases, nausea is due to an infection that may be contagious.