Same thing almost. Sorry little thing in our back garden for a day or two but had a collar so posted on the local Moo Baan LINE group but no response so took her to the vets. She had bacterial and viral infections, conjunctivitis rhino sinusitis and jus about everything. Despite the collar the vet she had never been vaccinated. She is still under treatment at the vets, day 5 but now improving rapidly. She will be given the required vaccinations and spayed when well enough.
Unlikely we will keep her although she is a real cute young Siamese-ish and seems loving and playful despite her illnesses. Not really cat people and age is also a factor.
Will look for a good home for her soon, maybe try the ladies at adopt meow Ch.