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- Free-living ameba: Acanthamoeba spp, Naegleria fowleri, Balamuthia mandrillaris, Sappinia pedata. 
- Ubiquitous in nature: isolated from air, soil, fresh water, salt water, chlorinated swimming pools, sewage, heating and ventilation systems.
- Two-stage life cycle
- Actively feeding & dividing trophozoites (14-40 μm in diameter)
- Dormant cysts (double-walled 10-25 μm in diameter); cysts resistant to chlorine, low temperature, antibiotics, pH extremes.
- Three groups (I, II, III) of Acanthamoeba spp described, division based on cyst morphology. 
- Encystation occurs under environmental stress: food deprivation, desiccation, change in temperature.
- Transmission: inhalation most common route; direct inoculation of skin
- Described as “Trojan horses” as they can harbor intracellular bacteria.