If a hamster is ill or injured, keep the hamster warm, and encourage it to take some food or water (by dropper if necessary) until a vet can be seen.
Abscesses: these are pockets of infection, which can form from fairly minor breaks in the skin. Pus accumulates under the skin, sometimes forming a sizable lump (which may sometimes begin draining on its own). Abscesses can form from cuts or scratches on the skin and also in the cheek pouches if abrasive food material causes scratches in the lining. If a hamster continually looks like it has food in packed in its cheek pouches, there may be an abscess or an impacted cheek pouch present. Abscesses require veterinary attention for draining, flushing, and treatment with antibiotics.
Respiratory Infections: hamsters can get respiratory infections that can lead to pneumonia. Signs include sneezing, discharge from eyes or nose, wheezing and labored breathing. Occasional sneezing is not too worrisome, but if there is any loss of appetite, decreased activity, wheezing or difficulties breathing, immediate veterinary attention should be sought.
Wet Tail: also called proliferative ileitis and regional enteritis. This is a highly contagious disease, and most common in recently weaned hamsters. The cause is uncertain, but a bacteria called Campylobacter jejuni may be involved, and in some cases the disease is associated with stress, crowding, and diet changes. Affected hamsters may die very quickly, exhibiting signs such as diarrhea (causing wetness around the tail), lethargy, loss of appetite, and ruffled coat. Note: not all hamsters with diarrhea have this disease, see below.
Diarrhea: a number of infections can cause diarrhea (including but not limited to wet tail), along with diet changes and treatment with antibiotics. Over feeding vegetables and other fresh foods is a fairly common cause of diarrhea, but in this case there is usually no loss of appetite or decrease in activity. Dehydration is a real concern, so make sure the hamster is drinking if diarrhea occurs, and a veterinarian should be consulted. With diarrhea, withhold fresh foods for a few days and resume only if the diarrhea is completely resolved, and start back onto fresh foods slowly.
Skin Diseases: hamsters can be infested with a number of mites, which can be identified by a skin scraping by a vet and treated accordingly. Ringworm (actually a fungal infection) can also occur, and requires treatment by a vet. Allergies and skin infections can also occur. Hair loss is not all that unusual and can be seasonal or happen in older hamsters. If there is flakiness or redness of the skin or any lesions on the skin, or the hamster appears to be itchy and scratching more than usual, a vet should be seen. Hamsters do have scent glands on their flanks which can be dark and sometimes alarm owners. These occur on both sides of the body and do not appear irritated or bother the hamster. Cedar bedding can also cause skin irritation or allergies in addition to lung problems so should be avoided.
Hibernation: if the room temperature is allowed to drop below normal room temperature, hamsters will go into a sort of hibernating state, where they are very still and breathe very slowly. Many owners panic and think their hamster is dying or dead, when might be a matter of the hamster getting too cold (for exapmle if the owners go away for the day and turn the thermostat down in the house). Rewarming the hamster should be sufficient.
Important Note About Cedar Bedding: cedar emits volatile compounds which are irritating to the respiratory tract (and may also cause changes in the liver). Pine shavings also release volatile compounds to a lesser but still significant extent. These beddings (particularly cedar) may also cause a skin sensitivity or allergy, and should be avoided. Aspen shavings or some other hardwood shavings are a better, safer option. See "The Problem with Cedar Shavings" and "Top Ten Alternatives to Cedar and Pine."
Antibiotics: several antibiotics cause a fatal toxicity in hamsters. Penicillin, amoxicillin, ampicillin, streptomycin, dihydrostreptomycin, tetracyclines, lincomycin, erythromycin, vancomycin, cephalosporins, and gentamycin should be avoided.