To address the aim of this qualitative review, the authors performed a literature search using Pubmed/MEDLINE databases using the terms: “Biomarkers”, “Procalcitonin” and/or “C-Reactive Protein”, in combination with search terms including “Influenza Pneumonia”, “SARS-CoV-2 pneumonia”, “COVID-19”, “Bacterial co-infection”, “Mixed viral bacterial co-infection”, “Antimicrobial stewardship”, “Mortality”, “Prognosis”. The gene discussed is CRP; the disease is coinfection.