The relationship between infectious diseases and COVID-19: Co-infections, immune dysregulation, antimicrobial resistance, and clinical outcomes

SARS-CoV-2 has brought about an unprecedented change in the global landscape of infectious diseases. In addition to its own respiratory pathology, COVID-19 patients are vulnerable to a wide variety of secondary bacterial, fungal, and viral co-infections, with multiple mechanisms of convergence: massive lymphopenia; defect in macrophage and neutrophil function; disruption of the mucosal barrier; corticosteroid therapy; and use of invasive supportive therapies. This review comprehensively examines the bidirectional relationship between COVID-19 and infectious diseases, analyzing the epidemiology and microbiology of bacterial co-infections caused by Staphylococcus aureus, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Acinetobacter baumannii; fungal infections including COVID-19-associated pulmonary aspergillosis (CAPA), Candida spp., and the unprecedented mucormycosis epidemic; and viral co-infections including influenza and herpesvirus reactivation. Host immune dysregulation—the central facilitating mechanism—is examined at molecular and cellular levels. The review further addresses the antimicrobial resistance crisis catalyzed by the pandemic through empirical antibiotic overuse, disrupted stewardship, and nosocomial multidrug-resistant organism outbreaks. Diagnostic advances, therapeutic strategies, and public health implications are discussed with emphasis on low- and middle-income settings.

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Introduction

The COVID-19 pandemic, declared by the WHO in March 2020, resulted in over 700 million confirmed infections and more than 7 million deaths globally by 2024. While SARS-CoV-2 itself causes a spectrum of disease from asymptomatic infection to fatal acute respiratory distress syndrome (ARDS), the clinical course of severe COVID-19 is substantially shaped by secondary infections. This pattern was set by history during the 1918 Spanish flu pandemic when most deaths were due to secondary bacterial pneumonia, not the influenza virus. COVID-19 is a recapitulation and extension of it with further complexity. [1,2] The link between COVID-19 and infectious diseases is two way and multi-dimensional. The mechanisms by which SARS-CoV-2 actively undermines innate and adaptive immune defenses include interferon antagonism, induction of lymphopenia and neutrophil dysfunction, which result in a permissive immunological environment for co-pathogens. At the same time, the global pandemic has led to unprecedented empirical antibiotic prescriptions, disturbed antimicrobial stewardship, extended hospitalizations and a healthcare system overwhelmed, all of which have had a significant impact on the global antimicrobial resistance (AMR) crisis. [3,4] This review aims to provide a summary of the existing evidence on all aspects of coinfections associated with COVID-19 infection, including the immune basis, clinical consequences, diagnostic and therapeutic approaches, as well as public health implications, with special emphasis on the accompanying infections in Iraq and the Middle East region. [5] unit. In a ground-breaking systematic review, Lansbury et al (2020) combined 30 studies that included 3,834 patients, and found that the prevalence of co-infection was 6.9% at the time of admission and 14.3% in patients admitted to the ICU. About 72% of the reported co-infections are bacterial, 19% fungal, and 9% viral, but these figures may be significantly under-estimates because of antibiotic pretreatment and inter-variant differences in testing practice. [6,7] There is a high degree of geographical variation. Iraq, Iran, and India were among the low-and middle-income countries that had significantly higher rates of fungal coinfections, especially mucormycosis, due to high prevalence of diabetes, overuse of steroids, and poor health care systems that control infections. Factors associated with risk of co-infections are mechanical ventilation, central venous catheterization, broad-spectrum antibiotics, diabetes mellitus (which causes a decrease in chemotaxis, phagocytosis and oxidative burst of neutrophils, and supplies the glucose for fungal growth), and corticosteroid administration. [8,9] Table 1: Summary of major co-infections in COVID-19: pathogen, prevalence, and clinical impact. [10][11][12] Type Key Pathogen(s) [15,16]

Type	Key Pathogen(s)	Prevalence in COVID-19	Main Clinical Impact
BACTERIAL
Bacterial	Staphylococcus aureus / MRSA	12-21% severe cases	Secondary pneumonia, MRSA bacteraemia, toxic shock
	Pseudomonas aeruginosa	6-18% ICU	VAP, XDR infections, prolonged ventilation
	Klebsiella pneumoniae	5-14%	Septicaemia, ESBL/CRE emergence, multi-organ failure
	Acinetobacter baumannii	4-10% ICU	Carbapenem-resistant VAP, outbreak clusters
FUNGAL
Fungal	Aspergillus fumigatus (CAPA)	3-33% ICU/steroid Rx	Invasive pulmonary aspergillosis, ~47% mortality
	Candida spp. / C. auris	5-26% ICU	Candidaemia, nosocomial outbreaks, MDR C. auris
	Mucor / Rhizopus spp.	~0.1-1.8% (India epidemic)	Rhinoorbital-cerebral mucormycosis, 35-54% fatality
VIRAL
Viral	Influenza A/B	3-20% early pandemic	Additive respiratory failure, higher mortality
	CMV / EBV reactivation	20-30% ICU patients	Interstitial pneumonitis, immunosuppression amplification

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Summary of major co-infections in COVID-19: pathogen, prevalence, and clinical impact. [10-12]

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Klebsiella pneumoniae and Other Gram-Negatives

Klebsiella pneumoniae is the second most common gramnegative co-pathogen, responsible for pneumonia, bacteraemia, and urinary tract infections in Echinocandins are first-line for candidaemia; C. auris infections require susceptibility-guided therapy. [23,24]

COVID-19-Associated Mucormycosis (CAM)

The CAM epidemic in India during April-June 2021 produced over 47,000 cases-a 100-fold increase above baseline-with rhinoorbital-cerebral mucormycosis (ROCM) predominating and case fatality rates of 35-54%. The pathobiological convergence of multiple COVID-19specific factors drives CAM: hyperglycaemia (both preexisting and steroid-induced) creates an iron-rich, acidotic environment optimal for Mucor and Rhizopus germination; corticosteroid therapy suppresses macrophage conidial killing; COVID-19-induced sinonasal epithelial disruption provides a portal of entry; and COVID-19-associated hyperferritinaemia supplies fungal iron acquisition. Treatment requires urgent surgical debridement combined with liposomal amphotericin B, with isavuconazole as an alternative. [25,26]

Viral Co-infections and Herpesvirus Reactivation

Influenza A/B co-infection with SARS-CoV-2-so-called 'flurona' was documented in 3-20% of early pandemic cohorts, with co-infected patients demonstrating greater respiratory failure severity and higher mechanical ventilation rates. Respiratory syncytial virus, parainfluenza, rhinovirus, and human metapneumovirus collectively complicate 1-8% of cases. Multiplex syndromic PCR panels simultaneously detecting SARS-CoV-2 alongside influenza and RSV from a single nasopharyngeal swab enable pathogen-directed therapy within 1-2 hours and reduce inappropriate antibiotic use. [27,28] Reactivation of latent herpesviruses-particularly EBV, CMV, and HSV-occurs in 20-30% of COVID-19 ICU patients, substantially exceeding rates in comparable non-COVID-19 ICU populations. COVID-19-induced lymphopenia and T-cell exhaustion (PD-1↑, TIM-3↑, LAG-3↑) remove the immunological restraint normally suppressing herpesvirus latency. CMV reactivation causes interstitial pneumonitis, hepatitis, and gastrointestinal disease in severe COVID-19 patients, contributing to the immune dysfunction enabling bacterial and fungal superinfections. EBV reactivation, documented in 30-60% of severe cases, has been proposed as a contributing mechanism to long COVID symptomatology. [29,30] 6. Host Immune Dysregulation: The Facilitating Mechanism 6.1 Innate Immune Suppression SARS-CoV-2 encodes multiple interferon antagonists-NSP1, NSP3, NSP12, ORF3b, ORF6, ORF9b-that suppress type I and III interferon production and signaling at multiple levels, creating a window of innate immune vulnerability paradoxically profound in severe disease. COVID-19 simultaneously shifts alveolar macrophages from antimicrobial M1 toward anti-inflammatory M2 phenotype, markedly reducing phagocytic killing of bacteria and fungi and creating a permissive pulmonary microenvironment. [31,32] 6.2 Lymphopenia and Adaptive Immune Failure Lymphopenia (count <1.0 × 10⁹/L) occurs in 83% of hospitalized COVID-19 patients and is the strongest independent predictor of severity and mortality. SARS-CoV-2 depletes CD4+ T helper, CD8+ cytotoxic T, and NK cells through direct lymphocyte infection, apoptosis induction, and lymphoid sequestration. The resulting functional exhaustion-characterized by PD-1, TIM-3, and LAG-3 upregulation-resembles sepsis-induced immunosuppression and HIV-associated immunodeficiency, explaining susceptibility to opportunistic pathogens including Pneumocystis jirovecii and Cryptococcus neoformans typically associated with severe immunocompromised. [33]

Cytokine Storm and the Therapeutic Paradox

Massive IL-6, IL-1β, TNF-α, and IL-8 release in the COVID-19 cytokine storm damages respiratory epithelium and intestinal mucosa, creating microbial invasion portals, while driving the compensatory anti-inflammatory response that produces profound subsequent immunosuppression. Therapies designed to suppress cytokine stormdexamethasone (WHO-recommended standard of care for severe COVID-19 requiring oxygen), tocilizumab (anti-IL-6R), baricitinib (JAK inhibitor)-while reducing COVID-19 mortality, simultaneously suppress immune effectors required to resist secondary pathogens. This therapeutic paradox demands individualized immunosuppression balancing cytokine storm prevention against infection susceptibility, with antifungal prophylaxis warranted in high-risk patients receiving prolonged corticosteroids. [34,35] Table 2. Key immune dysregulation mechanisms in COVID-19 facilitating secondary bacterial, fungal, and viral infections. [36,37] Broad-spectrum empirical carbapenem, cephalosporin, and fluoroquinolone use in COVID-19 ICUs selected ESBL, carbapenemase-producing, and MDR phenotypes at unprecedented speed. [2,38]

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Key immune dysregulation mechanisms in COVID-19 facilitating secondary bacterial, fungal, and viral infections. [36, 37]

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Mechanism

Mechanism

Disruption of Antimicrobial Stewardship

Antimicrobial stewardship programs (ASPs) that had achieved significant pre-pandemic AMR reductions were severely disrupted during surge periods: ASP personnel were redeployed; audit/feedback activities ceased; and preauthorization requirements were relaxed. European and American centres documented 20-40% increases in ESBL, MRSA bloodstream infections, and Clostridioides difficile infection in the first pandemic year. Candida auris outbreaks confirmed by whole-genome sequencing-in COVID-19 ICUs, alongside XDR K. pneumoniae, A. baumannii, and VRE outbreaks, represent the AMR consequences of overwhelmed infection control infrastructure. [39] Table 3: Impact of the COVID-19 pandemic on antimicrobial resistance: mechanisms, documented consequences, and evidence. [40,41] [41,42]

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Impact of the COVID-19 pandemic on antimicrobial resistance: mechanisms, documented consequences, and evidence. [40, 41]

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AMR

AMR

Antibacterial and Antifungal Management

Empirical antibiotic therapy should be based on local resistance patterns and severity of the infection, and should aim to cover S. pneumoniae and S. aureus in communityacquired pneumonia and gram-negative organisms, such as P. aeruginosa, in ICU-onset infections, and then be deescalated upon culture data. Novel beta-lactam/inhibitor combinations (ceftolozane-tazobactam, ceftazidimeavibactam) provide extended MDR gram-negative coverage; MRSA VAP should be treated with vancomycin, using AUC monitoring, or linezolid. Voriconazole or isavuconazole are being used as first-line antifungals in COVID-19 patients with high-risk features in ICUs who are on prolonged corticosteroid use, where antifungal prophylaxis may be indicated; echinocandins are being used in candidaemia; and liposomal amphotericin B is indicated in mucormycosis, where surgical debridement is urgently required. [43] 9. Conclusions and Public Health Implications COVID-19 and infectious diseases have a complicated relationship that is bi-directional and clinically relevant. SARS-CoV-2 generates a complex immune deficiency: interferon suppression, lymphopenia, barrier damage by cytokine storm, and iatrogenic immunosuppression (allergic and autoimmune deficiencies), leading to an incredible spectrum of secondary bacterial, fungal, and viral infections. The pandemic led to an exponential increase in the global AMR crisis, with huge numbers of empirical antibiotic prescriptions given, disruption to stewardship, and nosocomial outbreaks of MDR in overcrowded ICUs. [44] These challenges are especially significant for LMCs such as Iraq, where baseline AMR is high, and the prevalence of fungal disease and diabetes is high; as well as in which access to novel antifungals (isavuconazole, liposomal amphotericin B), advanced diagnostics, and second-line antibiotics is limited. AMR surveillance strengthening, ASP and IPC sustaining during public health emergencies, essential antimicrobial supply chains, and investing in infrastructure for swift diagnostics must be part of pandemic preparedness planning. [45] Lessons from pandemic co-infection should guide future preparedness for health emergencies: Microbiological sampling before starting empirical therapy, pathogen-driven de-escalation, ongoing ASP activity, strong IPC even during a surge, and equitable global access to diagnostics and therapeutics are non-negotiables of an integrated response to the intersection of COVID-19 and the global burden of infectious diseases. [46]

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