Bacterial vs Viral Infections: Similarities and Differences 

By Acharya Tankeshwar •  Updated: 07/05/22 •  5 min read

Bacterial infections are caused by bacteria whereas viral infections are caused by viruses. 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 viral infections are caused by viruses. 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. Severity and outcome of viral infections are affected by a number of 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 in 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 also increases the severity of the disease which otherwise is mild in the immune-competent hosts.
    4. Increased levels of corticosteroid suppress various 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 to control 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 using 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 aid in the diagnosis of bacterial infections.
  8. Serologic tests and polymerase chain reactions (PCR) assays are also useful for the diagnosis of viral infections. Serologic tests are done for systemic bacterial infections whereas PCR assays are reserved for non-cultivable or hard-to-culture bacterial pathogens.
Difference between bacterial and viral infections

Differences between Bacterial and Viral Infections 

Bacterial InfectionsViral Infections 
Bacterial infections are treated with antibiotics. Antimicrobial susceptibility testing is done to select appropriate antimicrobial agents for the treatment.Antibiotics are ineffective to control viral infections. Antiviral agents are used to treating viral infections. 
Bacterial infections can be both symptomatic as well as asymptomatic. Although the infection is asymptomatic initially, it may later manifest as a disease. Asymptomatic carriers of bacteria can transmit diseases to other individuals. For example, cholera, typhoid, and tuberculosis.Most viral infections are asymptomatic. Even if symptomatic, they are mild.  
Effective antimicrobial therapy is required for the treatment of symptomatic bacterial infections. Most viral infections resolve spontaneously. For example, adenovirus infection and the common cold
Polymorphonuclear neutrophils (PMNs) numbers increase significantly during bacterial infections. However, in certain bacterial infections such as typhoid fever, a decrease in the number of leukocytes (leukopenia) occurs. Viral infections are lymphocyte-predominant. Macrophages, particularly fixed macrophages of the reticuloendothelial system and alveolar macrophages, are the important cell types in limiting viral infection. 
Bacterial meningitis is associated with low glucose concentrations in CSF.Viral infections of the central nervous system (CNS) have normal glucose levels. 
Only a few bacterial infections are associated with cancers. Bacterial infections associated with cancers are Helicobacter pylori and Campylobacter jejuniCertain viruses such as HPV, HIV, HBV, etc. can cause or predispose to cancer. Viruses account for 12% to 20% of cancer cases worldwide. 
Direct gram stain of the sample is an important step in the laboratory diagnosis of bacterial infection. Cytopathic effect (CPE) is an important initial step in the laboratory diagnosis of many viral infections. CPE is taken as a hallmark of viral infection of the cell but all viruses do not cause CPE.
Examples of bacterial infections include cholera, enteric fever, gonorrhea, syphilis, tuberculosis, leprosy, etc. Examples of viral infections include common cold, chickenpox, flu, measles, mumps, COVID-19, Japanese encephalitis, dengue, AIDS, etc. 

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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