if single sided discharge-may be abcess tooth or decayed...
but more like food probable causation/aggravtion--diet must be high quality (fresh 'kill' best--home prepared next choice)....
male?female?
spayed?neutered?
100% house cat?how much get outside?
where live ?country?city?..etc

what condition of hair?
what condition of nails? how much 'clawing ' observed?
what noticed concernig thirst?

what illnesses in past?when? how treated?
what vaccinations?when?