At certain times it may appear that your rat is sick or not feeling well. And some of the common more clinical sign you may notice may be vague such as loss of appetite, changes in normal routine and normal behavior, you may notice that they are spending more time sleeping, that they don't want to be disturbed, they don't wake up or come to you for certain treats, or there's just some obvious change in how they look and how they feel.
And these are certainly different than other more specific symptoms. In those cases you're dealing with your rat just not feeling right, just not being themselves, just telling you that something is wrong but they're not showing you something obvious, and this unfortunately is a common scenario with some rats.
The flip side to that maybe your rat shows obvious signs of an illness. One of the most common problems that effect pet rats is respiratory diseases in forms of pneumonia, bronchopneumonias that are caused by different bacteria, different viruses, and other groups of infectious agents. And in these cases your rat is going to show symptoms of sneezing, difficulty breathing, you may notice that they actually open their mouth to breath which indicates that there is a problem. Also, rats will often produce a red color pigment from their eyes and their nose. This is something that should not be confused with blood, it's actually a glandular secretion that's normal in rats. Normal for them to produce at times where they are sick, at times where their body is under stress. It is generally not something that they would normally produce so when people notice the red serection from the eyes or the nose or they notice their rat is wiping the secretion that is on their forelimbs it is also an indication that the rats body is under stress from some type of illness.
Another large category of illnesses that rats will sometimes show symptoms for are growths, or tumors, or masses. In this case you're going to be looking for lumps, or bumps, or swellings, or any type of obvious mass anywhere over your rats body and this is an important thing to notice because some of these may require surgery.
So, there are basically two categories. There are non specific symptoms like the rat is just not looking well and feeling well and maybe loosing their appetite. And there are specific symptoms. It's important to remember that rats cannot vomit, they are not physically able to vomit. They can passively spit up food, but vomiting is not a common problem. But if you notice that your rat has lost their appetite, their sleeping too much, their showing symptoms of having diarrhea, or they're exhibiting this red color pigment produced from their eyes or their nose, or having trouble breathing it's important that you take them to a veterinarian.