Germs (also called microorganisms or microbes) are tiny, invisible living organisms or infectious agents capable of causing disease in humans, animals, or plants. They include four main types: bacteria (single-celled prokaryotes, e.g., Streptococcus causing strep throat or E. coli in some urinary tract infections), viruses (non-living particles that require host cells to replicate, e.g., influenza virus, norovirus, or SARS-CoV-2), fungi (e.g., Candida causing thrush or dermatophytes causing ringworm), and protozoa (single-celled eukaryotes, e.g., Giardia from contaminated water or Plasmodium causing malaria). Germs spread through direct contact (skin-to-skin or sexual), droplets (coughing/sneezing), airborne particles, contaminated food/water, vectors (mosquitoes/ticks), surfaces (fomites), or fecal-oral routes. Many are harmless or beneficial (e.g., gut microbiome aiding digestion), but pathogenic ones can invade tissues, multiply, and trigger immune responses leading to symptoms like fever, inflammation, rash, diarrhea, cough, or systemic illness depending on the type and site of infection.