Getting a chest X-ray is the only sure way to diagnose pneumonia. But there are some clues to help determine if it is not a simple cold or the influenza virus. The secondary fever is the biggest hint that something more serious is going on, Hendley said.
Bronchitis can be difficult to diagnose, particularly in children. One of the hallmarks of the disease, following a few days of fever and a runny nose, is coughing up green or yellow phlegm.
"Kids will obligingly swallow their mucus," Hendley said, often without telling their parents what it looked like. Parents may not know that their children have a secondary infection.
But phlegm is indicative of this disease because the main air passages in the lungs, called the bronchioles, become inflamed and produce a large amount of mucus. Bronchitis can occur at any time of the year, although it tends to pop up most frequently during the winter months.
Chest soreness and congestion is also associated with bronchitis.
Pertussis is a bacterial disease characterized by spasms of violent, uncontrolled coughing. Between spasms, patients often take in sharp breaths that make a noise like a "whoop," which gives the disease it's other name: whooping cough.
"The first stage of [pertussis] for all the world is a cold," Hendley said. And if the pertussis is treated during the first stage, it likely will not progress. "The problem is, you just can't tell that. ... When they begin the paroxysmal phase, then you can tell."
But these coughing spasms, which begin about one week after the illness begins, can be quite bad, especially among children under six months because pertussis can be fatal. About 7,000 cases of pertussis are reported each year in the United States, according to the Centers for Disease Control and Prevention in Atlanta. And while young children often receive vaccines against pertussis, the CDC also reported that cases of the disease are on the rise among teenagers, with more than 20,000 cases each year.
Meningitis is an infection of the fluids in the spinal cord and around the brain. The disease can begin with headaches, high fever, stiff neck and and sensitivity to light.
Meningitis can be bacterial or viral but bacterial meningitis is more serious and can cause brain damage, hearing loss or learning disabilities. Early diagnosis and treatment is crucial to prevent permanent brain damage.
The disease is transferred through respiratory secretions, droplets sprayed when someone coughs, for example, or by kissing.
Shaffner said the relatively mild presentation of meningitis might lead many doctors to treat older patients with a pain reliever or a fever reducer and a note to return if their symptoms do not improve.
"[Meningitis] can be so sneaky that a child -- a young adult, a college student -- can be semi-comatose in the bed in five hours," Schaffner said.
"HIV virus can present with flu-like symptoms in the very beginning," Brownfield said. Fevers, blotchy skin, swollen lymph glands, fatigue and lethargy, or a dry cough can all herald an infection.
Brownfield said symptoms like these can persist for a long time -- six to twelve weeks -- before any changes occur that could offer a clue as to whether someone may have HIV. Developing a rash can be an indicator of something amiss, particularly if someone is at high risk for catching the virus, because rash is not usually co-morbid with the cold or the flu.