Protozoans are not viruses, they are like amoebas.
Illnesses such as protozoans/coccidia can sometimes not show up for months, so it could very well have been a pre-existing condition from before the geckos came into your care. This is why it is recommended to quarantine any new animals for a minimum of three months.
That being said, it is common for even captive-bred crickets to carry worms, which are the most common-and easily treatable- cause of gecko diahrrea, especially when the animals have not been exposed to potentials sources of illness like a new gecko that could be carrying protozoans.
You can bring a stool sample to a vet, they can easily spot worms with a regular fecal. Other illness caused by bacteria or protozoans can be more difficult to diagnose.
Do your gex have any other symptoms?