COVID-19

coronavirus disease 2019 (COVID-19) is an infectious respiratory disease caused by severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2), a betacoronavirus first identified in late 2019 that spread globally as a pandemic. Transmission occurs predominantly through respiratory aerosols and droplets, including airborne spread in poorly ventilated indoor settings. The virus enters host cells by binding its spike receptor-binding domain to angiotensin-converting enzyme 2 (ACE2), after which host proteases — furin during particle assembly and TMPRSS2 or endosomal cathepsins at entry — cleave the spike to release the fusion machinery; inside, the viral main protease (Mpro) and other nonstructural proteins process the polyproteins that build the replication complex. Because the receptor-binding domain is also the dominant antibody target, successive variants have accumulated substitutions there, which is why immune escape and vaccine reformulation have driven the epidemiology since 2021.

Clinical presentation ranges from asymptomatic infection through mild upper respiratory illness to pneumonia, acute respiratory distress syndrome, coagulopathy and multi-organ failure. Severe disease reflects dysregulated innate and adaptive activation involving interferons such as interferon gamma (Ifng), Interleukin-6 (IL-6) and other inflammatory mediators, with C-reactive protein and detectable circulating viral RNA used as severity markers — and the timing of that shift matters therapeutically, since the early phase is driven by replication and the later by the host response. A subset of patients develops persistent post-acute sequelae, or Long COVID, in which fatigue, cognitive complaints, chronic cough and airway hyperresponsiveness continue after viral clearance.

Treatment follows that division. Antivirals are given early: direct-acting inhibitors of the main protease, exemplified by nirmatrelvir with the pharmacokinetic booster ritonavir, and monoclonal or single-domain (VHH) antibodies against the spike receptor-binding domain, whose usefulness has repeatedly lapsed as variants escape them. Host-directed agents such as inhibitors of dihydroorotate dehydrogenase (DHODH), the rate-limiting enzyme of de novo pyrimidine synthesis, target a cellular dependency instead and so impose a higher barrier to viral escape. For patients already deteriorating on oxygen, benefit comes from damping the response rather than the virus — dexamethasone reduced mortality in that group, and interleukin-6 receptor blockade with tocilizumab adds to it. Prophylaxis rests chiefly on mRNA-based COVID-19 vaccines, which elicit spike-specific antibodies alongside CD4+ and CD8+ T-lymphocyte responses; protection is attenuated in immunocompromised groups, including transplant recipients, patients with hematologic malignancy or CD19-directed chimeric antigen receptor T cell therapy, people with chronic renal insufficiency, older adults, and individuals with conditions such as Down syndrome that carry intrinsic immune dysregulation. Beyond acute illness, the pandemic strained health systems worldwide, disrupted cancer, surgical, antiretroviral and mental health services, accelerated telemedicine, and reshaped vaccine technology and trial methodology.