What Are Diseases?

Diseases are abnormal conditions in living organisms that impair normal biological functioning, are characterized by specific signs and symptoms, and are distinguishable from injury or normal variation. They arise from a range of causes including genetic mutations, infectious agents, environmental exposures, immune dysregulation, and metabolic dysfunction. The study and management of diseases sits at the intersection of biology, medicine, and engineering, drawing on tools from pathology, epidemiology, molecular diagnostics, and biomedical instrumentation.

Disease classification has historically organized conditions by anatomical location and the nature of the pathological lesion, distinguishing, for example, neoplastic, inflammatory, degenerative, and infectious categories. The World Health Organization's International Classification of Diseases (ICD), now in its eleventh revision (ICD-11), provides the global standard for coding and reporting, expanding from approximately 14,400 entities in ICD-10 to more than 55,000 in ICD-11 to accommodate finer molecular and genomic distinctions. Modern classification increasingly incorporates molecular pathology, recognizing that diseases with similar clinical presentations can have distinct genetic signatures that predict different responses to treatment.

Pathology

Pathology is the scientific discipline that investigates the cellular and tissue-level changes caused by disease, providing the mechanistic foundation that links cause to clinical outcome. Anatomical pathology examines biopsied or autopsy tissue using microscopy and molecular staining to identify structural changes such as necrosis, fibrosis, and dysplasia. Clinical pathology encompasses laboratory medicine, including hematology, microbiology, and clinical chemistry, and produces quantitative measurements that guide diagnosis and monitoring. A key concept in cancer pathology is metastasis, the process by which malignant cells detach from a primary tumor, enter the bloodstream or lymphatics, and establish secondary tumors at distant sites. Understanding the molecular events governing metastasis is central to developing therapies that prevent disease progression. The PMC review of pathology as the clinical description of human disease outlines the disciplinary scope and its interface with clinical practice.

Epidemiology and Epidemic Spread

Epidemiology studies the distribution, determinants, and control of diseases in populations. It quantifies incidence (new cases per unit time) and prevalence (existing cases at a point in time), and identifies risk factors through cohort studies, case-control studies, and randomized trials. Epidemic modeling describes how infectious diseases spread through susceptible populations using compartmental frameworks such as the SIR model, which partitions a population into susceptible, infected, and recovered individuals and derives the basic reproduction number R0. When an epidemic spreads across multiple countries or globally, it is classified as a pandemic; the COVID-19 pandemic of 2020 demonstrated the critical importance of mathematical epidemiology in forecasting burden and evaluating non-pharmaceutical interventions. The PMC article on molecular pathological epidemiology and global precision medicine describes how integrating molecular data with population-level epidemiology enables more accurate causal inference and biomarker discovery.

Medical Diagnosis

Medical diagnosis is the process of identifying a disease from its signs, symptoms, and test results. It draws on clinical examination, medical imaging, laboratory assays, and increasingly on computational methods. Imaging modalities including X-ray, computed tomography, magnetic resonance imaging, and positron emission tomography reveal anatomical and functional abnormalities. Molecular diagnostics, including polymerase chain reaction (PCR), whole-genome sequencing, and immunoassays, detect specific pathogens, genetic variants, or protein biomarkers with high sensitivity. The PMC review of biomedical engineering techniques for tropical disease diagnosis surveys non-invasive diagnostic platforms including optical detection, lab-on-chip devices, and electrochemical biosensors developed for resource-limited settings.

Applications

Diseases have direct relevance across a wide range of engineering and scientific fields, including:

• Medical device development for patient monitoring, ventilation, and drug delivery
• Diagnostic imaging system design, including MRI, PET, and ultrasound instrumentation
• Vaccine and therapeutics development supported by computational modeling
• Public health surveillance infrastructure and outbreak detection systems
• AI-assisted pathology for automated tissue classification and prognosis