Bacterial vs Viral Infections: Similarities and Differences 

Bacterial infections are caused by bacteria, whereas viruses cause viral infections. Whatever the etiological agent, an infection in a person typically has four stages: 

1. incubation period: a period when the patient is asymptomatic, 
2. prodromal period: a period of non-specific symptoms, 
3. specific-illness period: a period when characteristics, symptoms, and signs appear, and 
4. recovery period: in this period, illness wane, and the patient regains good health.

While bacteria and viruses can both cause mild to serious infections, use similar modes of transmission, and have almost similar courses of infection, these infections also differ. The easiest difference anyone can figure out is bacterial infections are caused by bacteria, while viruses cause viral infections. Perhaps the most applicable difference is antibiotics are used to kill bacteria but ineffective for viruses.

Features of Viral Infections

1. Most viral infections are asymptomatic. Of those infections that are symptomatic, most are mild and self-limiting. Most viral infections are either localized to the portal of entry (common cold involves only the upper respiratory tract, influenza is localized in upper and lower respiratory tracts) or spread systemically throughout the body, for example, poliomyelitis, measles, etc. Systemic infections are the most serious viral infections.
2. The severity and outcome of viral infections are affected by several variables such as age, immune status, nutritional status, etc.
   1. Age is a significant variable in the outcome of viral infections. Generally, infections are more severe in neonates and the elderly than in older children and young adults.
   2. Malnutrition leads to more severe viral infections. For example, cases of measles infections in developing countries.
   3. Immunosuppression predisposes to various viral infections and increases the severity of the disease, which is mild in the immune-competent hosts.
   4. Increased corticosteroids suppress host defenses and predispose them to more severe viral infections, such as varicella-zoster virus, disseminated herpes virus infection, etc.
3. Types of viral infections
   • Disseminated or systemic viral infections: Examples include measles, mumps, rubella, and chickenpox
   • Local viral infection: For example, the common cold is a viral infection of the upper respiratory tract.
4. Though viruses are intracellular obligate parasites, both humoral immunity and cell-mediated immunity play active roles in controlling viral infection. Cytotoxic T cells kill the virally infected cells, and antibodies neutralize the viruses circulating in the body fluids such as blood and CSF. 
5. Viral infections are often diagnosed by PCR assay for viral DNA or RNA or by serologic tests for specific antibodies or detection of specific viral antigens.

You may also like to read: Differences Between Bacteria and Viruses

Features of Bacterial Infections

1. The initial step in the process of many bacterial infections is the adherence of the organism to mucous membranes. Many bacterial components such as pili (fimbriae), glycocalyx, capsule, lipoteichoic acid, etc. mediate the attachment process.
2. Most bacterial infections are acquired from external sources. However, some bacterial infections are caused by members of the normal flora when there are breaches in the anatomical barriers (e.g. trauma) or when the host immune status is weakened.
3. Most bacterial infections are communicable but some are not, for example, botulism and Legionella pneumonia.
4. Bacteria cause disease by two major mechanisms: toxin production (exotoxins and endotoxins) and invasion and inflammation.
5. Polymorphonuclear leukocytes (PMNs) are the predominant cellular defense in bacterial infection.
6. Host defenses against bacterial infections include both innate and adaptive immunity. Innate defenses are nonspecific and may include physical barriers, cells, and proteins. Adaptive (acquired) defenses are highly specific for the organism and include antibodies and cells such as CD4-positive helpers;
7. Microbiologic diagnosis of bacterial infections frequently is made using Gram stain and culture. Gram stain evaluation of direct smears is routinely used to diagnose bacterial infections.
8. Serologic tests and polymerase chain reactions (PCR) assays are also useful for diagnosing viral infections. Serologic tests are done for systemic bacterial infections, whereas PCR assays are reserved for non-cultivable or hard-to-culture bacterial pathogens.

Differences between Bacterial and Viral Infections 

References

• Madigan, M., Martinko, J., Stahl, D., & Clark, D. (2012). Brock Biology of Microorganisms (13th ed). Pearson Education
• Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition
• Review of Medical Microbiology and Immunology, Lange Medical Books, 13th edition