About one third of people globally carry a tiny parasite known as Toxoplasma gondii, which leads to toxoplasmosis. Following initial exposure the organism settles in muscle and brain tissue and may persist indefinitely. Although this lifelong presence might seem alarming, most infected individuals experience no symptoms and face no health issues unless pregnant or immunocompromised. The body typically manages such infections without notice. Attention to the parasite stems from its transmission routes and potential effects in vulnerable groups. Cats serve as the sole hosts where the organism reproduces and shed it in feces during first infection only. However infection also spreads widely through consumption of undercooked meat containing the parasite. Most cats show no signs after initial exposure and cease shedding thereafter. Pregnant women receive advice to avoid undercooked meat and cat litter because the parasite can cross the placenta and cause fetal harm including vision loss or miscarriage. Such outcomes remain uncommon in Australia with rates between 0.17 and 2.3 per ten thousand births so routine screening is not performed. Individuals previously exposed face no renewed risk during pregnancy. Severely weakened immune systems from transplants or HIV can reactivate dormant infections leading to serious illness though treatments exist. Retinal infection may cause gradual vision loss affecting roughly one in 149 Australians. Infection rates reach about fifty percent in France and eighty percent in Brazil where related complications occur more often due to dietary habits stray cats local strains and climate. After infection the organism resides in muscle or brain but tests detect only antibodies. In rodents it alters behavior to favor predation by cats completing its cycle yet human effects remain unclear. Studies link exposure to certain behaviors and disorders such as schizophrenia but these findings are correlational and require caution in interpretation.
