There are 3 infective stages of Toxoplasma gondii namely the oocyst, tachyzoite and bradyzoite.  Humans can acquire infection by:

1. Accidental ingestion of oocysts shed in the cats' faeces.

2. Ingestion, inoculation or placental transfer of tachyzoites

3. Ingestion of the bradyzoites (tissue cysts) in undercooked or raw meat or by organ transplantation.

1. Transmission of oocysts

Humans can become infected by accidentally ingesting oocysts, which are excreted in cat faeces.  Faecal contamination can be found in cat litter, soil and water.  Oocysts are very resistant and can survive for many months in soil or water.  Susceptible individuals, particularly pregnant women, should remove all traces of soil from fruit and vegetables and wash them thoroughly and care must be taken when disposing of cat litter and hand to mouth contact should be avoided when gardening.

2.  Transmission of tachyzoites

Humans can become infected by ingesting tachyzoites in milk although they tend to be killed by gastric juices  Susceptible individuals, in particular, pregnant women, should avoid the consumption of unpasteurised milk and milk products.  Infection can also occur through the transfusion of blood or blood products containing tachyzoites or through a laboratory needle stick injury.  Circulating tachyzoites in pregnant women may cross the placenta and infect the foetus.

3. Transmission of bradyzoites

Bradyzoites invade tissue (e.g. muscle and brain), replicate slowly and form tissue cysts. Humans can become infected by ingesting raw, undercooked or cured meat.  Susceptible individuals should thoroughly wash their hands and utensils when preparing raw meat and only eat meat which has been cooked thoroughly. 

Infection can also occur through transplant of an infected organ in a seronegative recipient.  Seronegative recipients of seropositive donor are given the appropriate prophylaxis.