Sudden changes in the skin or coat are a common sign of illness in dogs, according to WebMD. Vomiting and diarrhea can also be symptoms of many common health problems.
The type of changes to the skin can help narrow down the condition causing it, notes WebMD. Hot spots, which appear as suddenly irritated patches of skin or open sores, are often caused by allergies. If the dog's coat looks rough and poor in quality, especially if the dog is also losing weight, it may be a sign of parasites.
In addition to vomiting and diarrhea, a lack of appetite can be a sign that something is wrong with a dog, reports Canine Journal. Some conditions, such as diabetes, may also cause the dog to drink excessive amounts of water.
Excessive licking and scratching can also signify common health problems, explains WebMD. Dogs may lick or chew at itchy spots caused by fleas or allergic reactions. They may also lick at painful areas, such as joints suffering from arthritis.
Coughing is another symptom of many illnesses, including kennel cough, which is typically a mild bacterial infection. However, a mild but persistent cough can also be a sign of heartworm infection, which can be fatal, reports the American Heartworm Society.