Blood pressure rises when you have a fever because the heart rate increases during a fever, which causes a rise in blood pressure. Blood pressure measurements are affected by a variety of factors, such as emotional excitement, stress, exercise, etc. Hypertension is considered to be present if the blood pressure reaches or exceeds 140 mmHg systolic and 90 mmHg diastolic on at least three non-same-day occasions using the correct standardized measurements in conditions of quietness, sobriety, and the absence of antihypertensive medication. If only the systolic blood pressure reaches the standard, it is called simple systolic hypertension. The vast majority of hypertension is primary hypertension, and about 5% is secondary to other conditions called secondary hypertension, such as chronic nephritis and renal artery stenosis.