It is important that you have a rat-savvy vet in your area. Rats are classed as ‘exotic’ pets and so contacting any exotic vets is a good place to start. However, ‘exotic’ covers anything from lizards to parrots to axolotls so it’s important that you chat to the practice to make sure the vet is specifically experienced with rats.

Common illnesses include:

Illness Common treatments
Upper Respiratory Infection Usually treated with antibiotics like Baytril or Doxycycline. Steroids may be added if the infection is bad or anti-inflammatories like Metacam. Some vets are keen to nebuilise however this can be very stressful for rats so should be a last resort.
Sprain Cage rest in a hospital cage for a few days. Can be given pain relief such as metacam if it is a bad sprain.
Abscess A warm compress can be applied to lift the scab and help draw out the pus. Should be kept clean so it can heal from the inside out.
Cuts & bites Should be kept clean with diluted hibiscrub or saline solution. Usually they look much better within 24 hours. Vet treatment is needed if it becomes infected.
Pyometra This is bleeding or pus from the vagina and indicates a uterus infection. Antibiotics and a spay are require ASAP.
Tumour Tumours in rats are rarely cancerous and usually benign. They can be removed whilst small via surgery. A second option is to wait until the tumour becomes large enough it is impacting the quality of life for the rat and have them put to sleep. Waiting too long can cause the tumour to ulcerate (split open) which is extremely painful.
Hormonal aggression This is more common in rats from a poor breeding background as there is a genetic factor. It can effect both males and females. A neuter or spay and careful reintroductions can fix the issue. If the issue persists then they can be tried with the opposite sex. There is also the option of a hormal implant called Suprelorin. This is not guarenteed to make a rat infertile however.

For more detailed information on illnesses, I recommend Isamu Rats or Ratguide (the latter is a useful resource to pass onto vets as it has dosage information and links to studies).