Depends on what parasites really.
Some do and some don't.
Its very hard to tell what parasite is killing your fish as well. Only sure way is to cut him up and examine them after death. No help that is after the fact tho.
Might not be a parasite, could be bad diet? I don't know your particular fish, but if they eat veggies (peas, cucumber, zuccini etc..) well this might un bung them if there constipated.
Quarantine would be best tho. Just in case. Its very hard to treat internals because medication ingested by fish thru the water sometimes just won't cut it. Also if it does turn out to be an infection best not infect the others.
Good luck