## Infective Stage of Entamoeba histolytica **Key Point:** The cyst is the only stage capable of surviving outside the host and is the sole infective form transmitted via the fecal-oral route. ### Life Cycle Stages | Stage | Location | Viability | Infectivity | Role | |-------|----------|-----------|-------------|------| | Trophozoite | Intestinal lumen & tissues | Dies rapidly outside host | Non-infective | Causes disease; reproduces by binary fission | | Cyst | Intestinal lumen | Survives months in environment | **Infective** | Transmitted via contaminated food/water | | Precyst | Intestinal lumen | Unstable intermediate | Non-infective | Transitional form during encystation | | Metacyst | Theoretical only | N/A | N/A | Not a recognized stage in E. histolytica | **High-Yield:** Cysts resist gastric acid and bile, survive desiccation, and remain viable in cool, moist environments for weeks to months. This is why E. histolytica is endemic in areas with poor sanitation and contaminated water supplies. **Clinical Pearl:** Trophozoites are fragile and destroyed by stomach acid; only cysts reach the colon intact. Ingestion of as few as 10 cysts can establish infection. **Mnemonic:** **CIT** — **C**yst is **I**nfective, **T**rophozoite is not (for transmission).
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