Flu or “influenza” is a type of viral infection which affects both upper and lower respiratory tracts. Common symptoms of flu include fatigue, fever and respiratory congestion. It is highly contagious and affects primarily the respiratory system. Flu is typically caused by influenza A viruses belonging to strains H3N2, H2N2, H5N1, H7N7, H1N2, H9N2, H7N2, H7N3, H10N7, H7N9 and HIN1, two types of influenza B viruses and rarely by one strain of influenza C virus. Influenza often mimics the symptoms of common cold and in severe cases may lead to pneumonia and septicaemia.
Antibiotics are ineffective in treating influenza as because it is a viral disease. However, influenza may be managed by vaccines and antiviral agents. The influenza viruses inhibit the formation of ACTH hormone, which results in decreased levels of cortisol. Since, the immune system is not depressed by cortisol; there is a formation of pro-inflammatory cytokines and chemokines. These chemicals help to combat the viral infection and are also responsible for the fever, and headaches associated with influenza.
Upper respiratory infection (URTI) is an infection that occurs either in the pharynx, sinuses, and tonsils or in the middle ear. These infections occur in and around the tissues mentioned above. URTI may be caused by virus, bacteria or fungus. The symptoms of URTI include throat pain, headache, pain around the nasal cavities, pain in ear, running nose and fever. The lymph glands are often swollen in case of severe infections. If untreated URTI may lead to septicaemia or infection of the blood, asthma and bronchitis. Diagnosis includes blood tests for routine culture, detection of C-reactive protein levels and X-ray of paranasal sinuses. The most common bacterial species include Streptococcus. Antibiotics are mainly administered if such infections are caused due to bacteria or fungi. For symptomatic relief mast cell stabilizers and antihistamines are used to reduce the release of histamine, which will help in allergic manifestations (for example, running nose). A comparison between flu and URTI is presented below:
| Flu | URTI | |
| Organ system affected | Respiratory tract both upper and lower | Infection that occurs either in the pharynx, sinuses, and tonsils or in the middle ear |
| Period of Illness | Prolonged beyond 3 days | May be acute (less than 5 days) or chronic (beyond 1 year). For example, sinusitis |
| Contagious | Highly contagious | Contagious but not as high as FLU |
| Causative Pathogens | Exclusively viral | May be viral, bacterial or fungal |
| Microbes Involved | Influenza A viruses belonging to strains H3N2, H2N2, H5N1, H7N7, H1N2, H9N2, H7N2, H7N3, H10N7, H7N9 and HIN1, two types of influenza B viruses and rarely by one strain of influenza C virus | Common bacterial species involved are Escherichia Coli, Staphylococcus, and Streptococcus. The viral strains involved are Adenovirus while fungal strains are Chlamydia sp. |
| Incurred through | Sneezes, coughs and inhalation | Allergens, coughs, sneezes and heightened or lessened immune system |
| Symptoms | fatigue, fever, running nose(rhinitis) and respiratory congestion | difficulty in breathing, dry hacking cough, running nose and fever |
| Patho-physiology | Suppress ACTH and cortisol, leading to hyperactivity of immune system | Pathogens cause inflammation and due to super infection, immune system may be weak |
| Vaccination | Specific vaccination like trivalent or tetravalent flu vaccines that provides protection against influenza A and influenza B strains of viruses | No specific vaccination available as because the disease can be of bacterial, viral or fungal. |
| Therapeutic Intervention | Primarily vaccination | Use of antibiotics like penicillin and cephalosporin.steroids and anti-histamines may be administered |
| Seasonality | Highly seasonal | Can occur anytime during a year |
| Complications | Heart failure, diabetes and asthma | Untreated URTI may lead to septicaemia or infection of the blood, asthma and bronchitis |
| Susceptible Population | Pregnant women, Individuals infected with HIV, diabetics. | Individuals exposed to pollution, passive smoke and airborne spread from infected persons, and immune-compromised individuals |
| Prevention strategies | May not be prevented, however avoiding contact with affected persons may act as a precautionary measure | avoiding smoke, pollution or contact with infected persons. |