It's a wee bit complicated:

The pathogen is persistent. The oocyst (transmission form) can survive in soil for a very long time (months or even years).

It's also very small and can be carried by wind or water etc.

Standard washing and disinfecting of food has no effect on the oocyst. The the oocyst are strongly protected and remain dormant until inside a host.

The host range for the pathogen is unknown. It is found very rarely in animals so adequate screening is very difficult.

Genetically it undergoes sexual reproduction. This means that the population is naturally heterozygous and constantly changing. Unlike bacterial or viral infections you can not isolate a strain and link infections. This makes determining a source extremely complicated.

Most cases of infection are theorized to be mild.