Determining whether an illness like influenza stems from a virus or bacteria can be confusing. These microscopic entities cause various symptoms, but their fundamental biological differences dictate how they behave and, importantly, how they are treated. This article clarifies the distinctions between viruses and bacteria, explaining why influenza is a viral illness and discussing potential bacterial complications.
What Are Viruses?
Viruses are small infectious agents, tinier than bacteria, typically ranging from 20 to 400 nanometers in diameter. They are not considered living cells because they lack the machinery to reproduce independently. A virus consists of genetic material, either DNA or RNA, encased within a protective protein shell called a capsid. Some viruses also possess an outer lipid membrane, or envelope, which they acquire from the host cell.
Viruses are obligate intracellular parasites, meaning they must infect a living host cell to multiply. Their replication involves attaching to a specific host cell, injecting their genetic material, and then hijacking the host’s cellular machinery to produce new viral components. These newly assembled viral particles are then released, often destroying the host cell in the process, to infect other cells.
What Are Bacteria?
Bacteria are single-celled microorganisms, much larger than viruses, with sizes ranging from 0.2 to 10.0 micrometers in diameter. Unlike viruses, bacteria are prokaryotic cells, meaning they have a well-defined cellular structure and can live and reproduce independently. Each bacterium is enclosed by a rigid cell wall, primarily composed of peptidoglycan, which provides shape and protection.
Inside the cell wall, bacteria contain a cell membrane, cytoplasm, ribosomes for protein synthesis, and a single, circular chromosome of DNA located in a region called the nucleoid. Many bacteria also contain small, independent pieces of DNA called plasmids, which can carry advantageous traits. Bacteria reproduce primarily through binary fission, a process where a single cell duplicates its genetic material and then divides into two genetically identical daughter cells.
Distinguishing Viruses from Bacteria
The differences between viruses and bacteria are fundamental, impacting their biology and how they cause illness. Viruses are non-cellular entities, essentially genetic material wrapped in protein, whereas bacteria are complete single-celled organisms. This structural disparity contributes to their vast size difference.
These differences directly influence treatment. Antibiotics are designed to target specific bacterial structures or processes, such as cell wall synthesis or protein production, making them effective against bacteria. However, because viruses lack these bacterial structures, antibiotics are ineffective against viral infections. Antiviral medications, conversely, are developed to interfere with specific stages of the viral life cycle, such as preventing the virus from entering host cells or replicating its genetic material.
Influenza: A Viral Illness and Its Complications
Influenza, commonly known as the flu, is a viral illness caused by influenza viruses. These viruses primarily infect the respiratory system, leading to symptoms such as fever, chills, cough, sore throat, runny or stuffy nose, muscle aches, headaches, and fatigue. While some people may experience vomiting and diarrhea, these symptoms are more common in children than adults.
Since influenza is caused by a virus, antibiotics have no effect on the influenza virus itself and should not be used to treat the flu. Using antibiotics inappropriately contributes to antibiotic resistance, making it harder to treat bacterial infections in the future. Instead, antiviral medications are available to treat influenza, including oseltamivir (Tamiflu), zanamivir (Relenza), peramivir (Rapivab), and baloxavir marboxil (Xofluza). These antivirals work by inhibiting the virus’s ability to replicate, potentially reducing the severity and duration of flu symptoms, especially when started within 48 hours of symptom onset.
Despite influenza being a viral infection, it can sometimes be followed by secondary bacterial infections. The influenza virus can damage the lining of the respiratory tract, making it more susceptible to bacterial invasion. Common secondary bacterial infections include bacterial pneumonia, ear infections (otitis), and sinusitis. These complications, being bacterial in nature, can be treated with antibiotics. For instance, Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus are frequently reported bacteria associated with secondary infections after the flu, and bacterial pneumonia resulting from such infections has been a significant cause of mortality during past influenza pandemics.