Heat treatment apparatus for inactivating viruses in blood includes a series of four tubing coils with connectors at the opposite ends of the series for connecting the tubing to a blood source and a blood destination, a microwave heating chamber arranged to receive the first coil of the series, a dwell chamber to receive the second coil of the series, a cooling chamber adapted to receive the third coil of the series, and a microwave reheating chamber to receive the fourth coil of the series, with the first, second, and fourth chambers employing microwave energy for heating the blood as it flows within the coils and providing a radiometer circuit for monitoring the temperatures of the blood in the first, second, and fourth coils to produce first, second, and fourth temperature signals in response thereto, which is responded to by a controller to control the energy producing means to impart a selected time/temperature profile to the blood flowing through the tubing and to deliver that blood at a selected delivery temperature.