You'll 100% encounter health issues in your time of owning rats, even if you have perfect husbandry. Below is a list of common health issues, some symptoms and warning signs, and what you can do to treat them.
Rats have extremely sensitive lungs. They're very susceptible to respiratory infections, pneumonia, and viral diseases that affect the respiratory tract, like SDAV. Respiratory issues are very common and are usually environmental, but genetics can also determine how susceptible they are to respiratory infections. Common environmental factors include smoke and fragrances in the air, high ammonia levels, dirty cages, dry air, etc. Respiratory issues can go downhill very quickly, it's recommended to see a vet soon after they appear. Mycoplasma pulmonis is a bacteria that commonly causes respiratory issues in rats and you can read more about it down below. Treatment depends on what kind of respiratory issue you're dealing with.
Viral infections include SDAV and Sendai, both coronaviruses that infect the respiratory tract. There are a few different kinds of viruses that can infect rats, though, which is why quarantine is very important for them. All rats should be quarantined for at least 2 weeks when placed in new environments. Viral infections can be very detrimental to mischiefs and colonies. Treatment and symptoms depend on the exact virus.
Tumors are also very common in rats. There are both genetic and environmental factors that are very important in determining whether or not your rats will develop tumors. Genetic factors are factors like parents or siblings developing tumors, while environmental factors are factors like exposure to carcinogens (excess sunlight or carcinogens in food). You can get tumors surgically removed, but there will most likely be a point in your rat's life where you have to decide "will the age and health of my rat affect the chances of survival from surgery?". You need to evaluate how the tumor is affecting your rat's quality of life. Rats shouldn't have to live with tumors that are huge, hindering their mobility, or getting irritated from dragging on the floor. Tumors steal the body's nutrients and blood supply.
Pituitary tumors are common in older females. They can be very devastating, and rats showing the symptoms of one should be euthanized. There is no effective treatment; it's expensive, has a very low chance of success, and usually just prolongs suffering. Symptoms include sudden behavioral changes, weird walking/movement, sudden weakness in limbs, and other neurological issues. Pituitary tumors can sometimes be confused for strokes, but the prognosis is about the same. If you suspect your rat has a pituitary tumor, see a vet soon, as they worsen fast. Rats with pituitary tumors usually die of starvation (as they become unable to swallow) or cardiac arrest.
Abscesses are pus-filled "pimples" that develop on the body, usually after injuries. If you're a rat owner, you'll most likely deal with one at least once. Luckily, most can be solved at home. If your rat has a facial abscess, you should go to the vet as there's a higher chance of risks associated with them. Do not try to pop abscesses. They can pop inwards and cause sepsis. Hot soaks will help the abscess come to a head. Then, you can gently try getting the pus out. If you need help, you should consult a vet or experienced owner.
Bumblefoot is easily prevented, but it's not too hard to treat unless it's severe. You'll notice lesions, redness, or swelling on the bottom of the foot. You can treat it at home, but you should try antibiotics if you notice the feet are infected. Prevent bumblefoot by keeping a clean cage with no open wires. Bumblefoot can take days to weeks to heal depending on severity.
All rats have mites; they're microscopic and we can't see them. It's only a problem when it flares up, similar to Mycoplasma. There are a lot of reasons mites can flare up, most notably stress or being brought in from bedding or going outside. You can freeze your bedding if you like, but it's not necessary. You'll notice if your rats have mites from random scabs, usually on the back or under the neck. If you notice lesions on the ears, nose, or tail, those are mange mites. You'll be able to see lice or eggs on the body, and if you squish them you'll see blood. These can all be treated the same way through Ivermectin/Harka-Mectin, Revolution, or Beaphar's anti-parasitic. Antibiotics won't be effective and should only be used for infected lesions. Do not use Bob Martin branded products.