Viruses are tiny infectious agents that can only replicate inside the living cells of a host organism (humans, animals, plants, bacteria), as they lack the cellular machinery for independent metabolism or reproduction. They consist of genetic material—either DNA or RNA—encased in a protein coat (capsid), and sometimes an outer lipid envelope derived from the host cell. Viruses are not considered living organisms by many definitions because they cannot grow, maintain homeostasis, or carry out metabolic processes on their own. They spread through various routes including respiratory droplets (e.g., influenza, SARS-CoV-2), direct contact or bodily fluids (e.g., HIV, herpes simplex), fecal-oral transmission (e.g., norovirus, hepatitis A), vectors like mosquitoes (e.g., dengue, Zika), or contaminated surfaces/food. Once inside a host cell, the virus hijacks the cell’s machinery to produce more viral particles, often killing or damaging the cell in the process, which triggers immune responses leading to symptoms such as fever, fatigue, cough, rash, sore throat, diarrhea, muscle aches, or more severe organ-specific effects depending on the virus type. Common human viruses include those causing the common cold (rhinoviruses), flu (influenza), COVID-19 (coronaviruses), chickenpox/shingles (varicella-zoster), measles, mumps, rubella, hepatitis (A/B/C), HIV/AIDS, Ebola, HPV (linked to warts and cancers), and many others.