In one sentence, describe what the following article is about:

Changes in the condition or appearance of a guinea pig's eyes are often an indication of infection.  Conjunctivitis (pink eye), for instance, which is a bacterial eye infection, tends to cause redness and inflammation on the eyelid and surrounding the eye, and can cause oozing or dripping of fluid from the eye. While ear infections are less common than eye infections in guinea pigs, check the ears for pus or other discharge.  Also, if your guinea pig appears to be going deaf, is constantly scratching its ears, is walking with a tilt or imbalance, or is walking in circles or rolling, you may be dealing with an ear infection. Guinea pig teeth grow constantly and are meant to be kept worn down to the proper length by the roughage they eat.  Due to heredity, improper diet, or another cause, some guinea pigs will end up with teeth that are too long and/or misaligned, which is known as malocclusion.  Malocclusion, in turn, can make eating difficult or even impossible, leading to a host of problems, or cause bleeding and possibly infections. If your guinea pig starts eating less than normal, start drooling far more than usual (sometimes called “the slobbers”), or is bleeding from the mouth, have it checked for malocclusion.  Most cases can be treated by filing or cutting down the teeth. Pneumonia is the most common cause of death in pet guinea pigs and is usually caused by a bacterial infection.  Treatment can be a challenge due to guinea pigs' intolerance for certain antibiotics, but early detection is the best way to increase the likelihood of recovery. Watch for unusual oozing or discharge from the nose; sneezing; difficulty breathing or wheezing; fever; weight loss; loss of appetite; change in attitude (depression); inflamed, crusty, or dull and sunken eyes; or lethargy.  Generally speaking, pneumonia should be ruled out any time your guinea pig displays common signs of illness. Your guinea pig's waste can tell you a good deal about its overall health.  A lack of frequent urine or stool production is an immediate cause for concern, as is evidence of blood in either.  Alternatively, excessive urination or diarrhea also often indicate a problem and should be addressed without delay. While cleaning your guinea pig's living space, familiarize yourself with the typical output and appearance of its waste.  Healthy guinea pigs are well-oiled machines when it comes to eating food and producing waste, so take deviations from the norm at either end as evidence of a possible illness.

Summary:
Keep an eye out for infections. Watch for a malocclusion. Be wary of pneumonia. Don't ignore the droppings.