Blood pressure varies with many conditions, and it is possible that blood pressure may rise when you have a fever, but in some more severe fevers, blood pressure may even decrease, so it should depend on what the specific person is like. If a person has a normal blood pressure, the ability to regulate blood pressure is normal, and the fever may be accompanied by a certain amount of sympathetic excitement, so that the blood pressure will appear to rise. However, if a person does not eat or drink or has vomiting or diarrhea when he has a fever, this will be accompanied by dehydration. Although there is sympathetic excitation, blood vessels will contract and blood pressure will rise, but the dehydration will lead to insufficient blood volume, and when the factor of insufficient blood volume dominates, there will be a decrease in blood pressure. Whether blood pressure increases or decreases when you have a fever should depend on the specific situation. Although there are factors that raise blood pressure, some people are able to experience a decrease in blood pressure.