Cryptosporidium is a parasite that is very common in young farm animals and can easily be passed to people.
Sixteen others are being monitored after regular attendance at the farm or being in contact with those who have.
Public Health Wales is continuing to work with Coleg Gwent, which has written to all students who may have been on the Usk farm in March.
It has also cancelled a series of upcoming open days with primary schools.
Investigations are continuing into the outbreak which causes diarrhoea, stomach pain, nausea and vomiting.
Healthy people will usually make a full recovery.
Seven people have tested positive following a parasite outbreak at Coleg Gwent farm in Monmouthshire.