If you have pet rats, respiratory disease is not a maybe

Rats are fantastic pets and this is the tax on them. Nearly all pet rats carry Mycoplasma pulmonis. It's not a sign you bought from a bad breeder, it's effectively universal in the pet population and usually passed from the mother. Plenty of rats carry it and look completely normal for a long stretch. Then age, stress, bad air quality or another infection tips the balance and you get the familiar picture: sneezing, a red brown crust around the nose and eyes, clicking or wheezing on the breath, hunching, weight loss. The crust throws people, so worth saying: that red stuff is porphyrin, not blood. It's a stress and illness signal, and a little is normal, a lot isn't. This gets managed, not cured. Vets treat flares with antibiotics, often doxycycline plus enrofloxacin, sometimes long courses, sometimes a nebulizer. Rats treated early do a lot better than rats you watched for a week to see if it settled. The part you control is the environment. Ammonia off urine wrecks their airways and accelerates all of this, so bedding gets changed more often than feels necessary and ventilation matters, which means a wire cage rather than an aquarium. No cedar, no raw pine, nothing dusty. Aspen, kiln dried pine or paper. Two more things worth knowing going in. Rats are intensely social and shouldn't be kept alone, so same sex pairs or groups, always. And mammary tumors are very common in females, usually benign and usually removable if you find them while they're small, so run your hands over her once a week. Spaying reduces that risk substantially and is worth raising with a vet who sees rats regularly. They live about 2 to 3 years, which is the real downside of rats. Everything above is about making those years good ones.