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Bacteria: Very small, single-celled organisms, abundant in various environments, including the human body. While some bacteria can cause diseases, others play essential roles in digestion and food production.
Viruses: Much smaller than bacteria, viruses can only replicate inside living cells. They cause a wide range of human infections, from the common cold to serious conditions like AIDS. Unlike bacteria, viruses are not susceptible to antibiotics.
Pathogenic Fungi: Including moulds and yeasts, these organisms can cause infections such as ringworm and thrush.
Protozoa: Microscopic organisms larger than bacteria, responsible for infections like malaria. Transmission can occur through ingestion, sexual contact, or insect vectors.
Worms: While not always microscopic, pathogenic worms can spread infections between individuals. Examples include threadworm and tapeworm.
Prions: Infectious protein particles, associated with diseases like (New) Variant Creutzfeldt-Jakob Disease (vCJD).