Most cats shed oocysts only once after infection and are then effectively immune. Within 5 days the shed oocysts become infectious to other animals and to humans. Some Toxoplasma organisms migrate from the cat's intestine to muscle and brain but usually remain dormant. The same migration can also occur in humans. In humans, toxoplasmosis symptoms may be flu-like, in most cases there are no symptoms.
Infection is more serious in individuals with poor immune response.
If such people contract toxoplasmosis for the first time it can run riot in the body with the immune system unable to keep it in check. In people who have previously been infected, the dormant tissue cysts are activated. Stress can also activate dormant cysts. Activated tissue cysts can produce millions of toxoplasmas which can lodge in the heart or brain and other major organs. They may show no symptoms themselves, but the foetus may suffer birth defects including blindness and brain damage.