Other conditions can sometimes cause similar symptoms, leading people to wonder if they really have the flu or a different condition. Haemophilus influenzae is a bacterium that was incorrectly considered to cause the flu until a virus was demonstrated to be the correct cause in 1933. This bacterium can cause lung infections in infants and children and can sometimes cause sinus or other infections. Other bacterial or viral infections, including the common cold, may cause symptoms similar to the flu. Sometimes with a runny nose, stuffy nose, and cough, it is hard to determine whether a cold or the flu is the cause of the symptoms, although the flu tends to produce higher fever than the common cold. Sometimes, allergies produce severe respiratory symptoms as well. Symptoms of certain complications of the flu, such as pneumonia, lead to similar symptoms.