Nausea and vomiting are not diseases. They are symptoms of another condition.
Nausea is the very unpleasant feeling of being about to vomit. Vomiting is the spitting up of the contents of the stomach. It's associated with a feeling of nausea and strong contractions of the abdominal muscles. Vomiting is different from regurgitation, in which one spits up stomach contents without feeling sick and without strong muscle contractions.
There are many things that can make a person feel nauseous or vomit. Common causes include:
Drinking too much alcohol
Prescription and nonprescription drugs
Fear or other strong emotions
Motion sickness, such as being seasick or carsick
Chemical toxins in the environment
Some possibly serious conditions that cause nausea and/or vomiting include:
Meningitis or encephalitis
Vomiting -- especially if it comes with diarrhea -- can cause dangerous dehydration. This is more likely to happen to children. When caring for a sick child, be alert to symptoms that mean the child needs water: dry lips and mouth, sunken eyes, rapid pulse, rapid breathing, and, in babies, a sunken look to the soft spot on the top of the head.
Sometimes the cause of nausea and vomiting is psychological. This doesn't mean the symptoms aren't real. A qualified clinical psychologist or psychiatrist can help. People who often make themselves vomit may have an eating disorder. This can be a life-threatening condition. If you eat a lot and then make yourself vomit, you need professional help.