Mousekeeping

Common Illnesses + Conditions

Mice are not immune to the effects of illness and their poor breeding causes them to very often have or develop diseases. Often these conditions are the result of genetics and mice are very predisposed to many illnesses.
Information on the health of mice is few and far between, but health resources for rats may help, as the same often applies for mice.

I AM NOT A VET!!! Please take me with a grain of salt, I'm simply speaking from experience.

What Do I Do When My Mouse Gets Sick or Hurt?

You can do one of two things.

You can seek care from a vet, but know mice live very short lives especially if they're storebought and your visit may be for naught. Mice also qualify as exotic pets, a term that honestly shouldn't apply to them or most common pets it applies to, so their treatment will often be more expensive than taking, say, a dog in for a checkup. I understand many cannot afford exotic pet vet care (I can't myself), and exotic pet vets are extremely rare in some places, like where I live for example. Do call your local vet(s) though and ask if they are willing to see a mouse. A vet visit may help a ton, like in the case of younger mice, or may not help at all, like in severe acute cases of injury or illness, with illnesses not caught until their later phases, or with older mice.

You can also choose to treat it at home. Many mouse illnesses can be treated at home by providing comfort, medicine, etc. Again, mice are normally the animal of choice for medicine testing, so you can use most human medicines on mice, just keep them at low doses (though know mice have a fast rate of metabolism), monitor them while they're on medication, and know its kinda difficult to forcefeed a mouse medicine (there's ways to do it though). If the medicine is topical, know mice WILL try to groom it off and will thus consume some of the medication.
A lot of the time, especially with older mice, you'll find there is nothing you can really do for them. Making their lives as comfortable as possible should be paramount in their later years. Keep your mouse as comfortable, painless, and as happy as they can be.

Human-Borne Illnesses

Mice are used in animal testing for a reason- in regards to their biology, they are quite similar to humans. This also means they can catch our diseases, just as easily as we catch theirs.

Mice can catch various parasitic, bacterial, and viral diseases from humans and vice versa. Normally, this won't be an issue though, unless your mouse is immunocompromised in which case they may be more prone to infection from human-borne illnesses.
Mice, especially those that are store-bought, are very susceptible to airborne bacteria that result from humans coughing or sneezing. Remember to wash your hands after coughing or sneezing and to cover your coughs and sneezes. Try not to handle mice when you're ill with a viral or bacterial infection, and if you have to, wash your hands before handling your mice.

Respiratory Disorders

Mice (especially those that are store-bought) are EXTREMELY prone to respiratory issues. No matter what you do, this just appears to be genetic. You will know a mouse has a respiratory problem if you hear them randomly make "sneezing"-like noises.
Genetic respiratory disorders you can't do a whole lot about. Some mice will always have their respiratory issue, even when in a space without allergens.
Bacterial/viral infections and allergies are the most common forms of respiratory problems in mice. You can tell the difference by the length of the disease- if acute/short, it was likely just a bacterial infection, but if prolonged/chronic, it's probably a genetic thing or an allergy (or an infection that they are struggling to recover from).

If your mouse has an allergy, chances are it will affect the respiratory tract. Mice will VERY often have wood and dust allergies, so wood shavings and dusty beddings should be avoided as beddings. Wood allergies also come with fur loss in more severe cases. Cleaning out a mouse's habitat often can help mitigate allergies, but sometimes there isn't much you can do.

Mental Illnesses

Depression + Heartbreak

Mental illness is one of the hardest things to see a mouse suffer. Unlike us, they simply don't know how to cope with it. Mice are social animals and their mental health is heavily reliant upon this. Females must always have a partner, be it another female or a neutered male.
Depression can form at any time, though normally is seen in mice who have lost a cagemate. You can do your best to mitigate the symptoms by offering them toys and giving them another buddy, but often times depression will devolve into heartbreak.

Heartbreak is effectively a very severe case of depression, and in mice it will normally come from the death of a beloved cagemate. Heartbreak can be deadly to mice! It is especially serious in older mice who are ill already. The bodies of mice with heartbreak will sometimes just seem to... give up. Heartbreak has massive consequences on a mouse's physical health; they may eat/drink less, stop running or playing outright, stop interacting or barely interact with other cagemates, show symptoms of stress. It's incredibly saddening.
Giving an older mouse with heartbreak a baby mouse companion is not a fantastic way to go, however. Preferrably, get them an older companion who is less likely to be hyperactive and get in their way. Sometimes, giving a very spritely young mouse to an older, depressed mouse will simply make things worse.

Stress

Mice can develop stress via being improperly cared for, being traumatized (by bad owners, the death of a cagemate, etc), from surgeries, or as a result of other illnesses.
Stress often causes a mouse to scratch at themselves or attack other mice, make them run or jump at the walls or ceiling of their habitat repeatedly, and in severe cases cause other physical illnesses. Mitigating stress varies on the source of it, but making sure your mouse has enough space, toys, a cagemate (if female or properly neutered), etc. are the best ways to go about it.

Neurological Disorders

Strokes

Especially in later life, mice are prone to strokes. If you're unlucky enough to see the stroke initially occur, it's an awful sight you'll wish to forget (I've unfortunately witnessed one); a mouse may have a seizure and/or go temporarily or permanently blind, and they will no doubt panic to the point that even if they are well bonded with you, they will BITE as if you are an imminent threat.
There is no treating a stroke. Mice post-stroke will very often suffer partial paralysis, blindness, loss of balance, waltzing/circling, head tilt, odd behavior, and impaired judgement. They also often lose awareness of gravity and become a fall risk if placed on high surfaces without something or someone to protect them.

Waltzing/Circling

Diabetes

Obesity

Cancer

Scoliosis

Wheel(y) Tail

Injuries