According to the Centers for Disease Control and Prevention (CDC), the term "flu" is used to describe infection from a number of different viruses, including influenza A, influenza B and H1N1. Pneumonia, by contrast, refers to a lung infection and inflammation from any cause. Many different kinds of bacteria, fungi and viruses can cause pneumonia, including flu.
Classic Flu Symptoms
According to the CDC, the classic symptoms of flu include fever, headache, muscle pain, dry cough, sore throat, runny nose and nasal congestion. In up to 40 percent of cases of "swine flu," or H1N1 flu, diarrhea, vomiting or both are also present.
Classic Pneumonia Symptoms
According to the National Institutes of Health (NIH), classic symptoms of pneumonia include cough, fever, shaking chills, pain with breathing or coughing, shortness of breath, loss of appetite and fatigue. These symptoms may be mild or severe. For example, temperature may be slightly above normal or very high. Shortness of breath may occur only upon exertion, such as climbing stairs, or it may be present even at rest. People with chronic cardiac and respiratory disease may always exhibit some of these symptoms. With pneumonia, they will be much worse.
Symptoms of Pneumonia Caused by Flu
According to the NIH, the flu virus is the most common cause of viral pneumonia in adults. Patients with pneumonia caused by flu will exhibit all of the symptoms described above. However, while bacterial pneumonias usually result in "productive" coughing, with yellow or green mucus, the cough associated with influenza is usually dry or may be associated with small amounts of blood in severe cases. Infants, the elderly and people with medical problems--especially pre-existing respiratory diseases--may be at special risk for developing pneumonia due to flu.
Secondary Bacterial Pneumonia
Bacterial pneumonia may be more likely to develop after cold or flu. One variety, called secondary bacterial pneumonia, usually affects only one lobe, or section, of the lung. The patient will usually have a history of recent illnesses with cold or flu. Patients who are hospitalized tend to develop infections with different kinds of bacteria than those who are not, so a doctor or other health care provider will usually ask about this.
Other Kinds of Bacterial Pneumonia
According to the NIH, bacterial pneumonia is the most common cause of pneumonia overall, accounting for over 4 million cases each year. Most cases in the United States are due to infection with Streptococcus pneumoniae, also called pneumococcus. Pneumonia is most common during winter. All people with bacterial pneumonia need to be treated with antibiotics; about 1-in-5 patients need to be treated in the hospital.
