Rats can get sick, even very fit and healthy ones and it can all happen rather quickly so a quick repsonse is needed.

I would highly recommend that you purchases 2 books - NFRS Handbook on Common Diseases of the Fancy Rat and Debbie Ducommun's Rat Health Care. Both are very good and can help you identify any possible problems.

I would also recommend reading Rat Guide which has lots of details about various issues and treatments.

The most common issues you will come across in rats are:
1. Upper respiratory disease
2. Lower respiratory disease
3. Pneumonia
4. Abesses
5. Barbering
6. Mycoplasma / Mycoplasmosis
7. Tumours

While not as common as other issues, these viruses can cause a lot of heartache for owners:
8. SDAV (Rat Coronavirus) - Article on the NFRS website
9. Sendai Virus - Article on the NFRS website

Lower respiratory disease
Respiratory disease is among the most common health issue in rats. Learning to recognize your rat’s normal healthy behavior can alert you to early signs of illness. In addition, seeking treatment at the earliest signs of illness (e.g. upper respiratory issues or porphyrin staining) can often increase life span and comfort for your rat.

Pneumonia
Following on from the lower respiratory disease, if left untreated can develope in to Pneumonia. This is an infection and inflammatory process of the bronchioles, alveolar spaces, and interstitial tissue of the lung parenchyma.

Signs of Pneumonia:
•Porphyrin (rust colored) stains about nose and/or eyes.
•Wheezing
•Small coughs
•Congestion
•Sneezing
•Increase in rapid breathing
•Labored breathing (use of abdominal muscles to breathe)
•Gasping.
•Hunched posturing
•Piloerection (ruffled, bristled fur)
•Lethargy
•Poor or loss of appetite.
•Panic type movement related to inability to get enough oxygen into lungs.
•Changes in behavior due to illness (e.g. nipping, biting, avoidance)
•May hear Rales (crackling, often sounds like Rice Krispies® in milk, or a bubbling sound) when listening to chest with stethoscope.
•May hear Rhonchi (loud rumbling indicating presence of thick fluid) with or without a stethoscope.
•If significant consolidation present in lungs breath sounds may be diminished even when listening with a stethoscope.
•Presence of head tilt if otitis media/interna (ear infection) is present.
•Presence of secondary illness (e.g., tumors).

Pneumonia is the result of primary or secondary infections by organisms such as bacteria, viruses, or fungal and parasitic infections. These organisms may reach the lungs through droplet (aerosolization), contact, or hematogenous (in the blood) spread. Pneumonia can also result through the inhalation of chemical irritants, or due to the aspiration of food or of drugs being administered.
The inflammation, swelling, congestion or consolidation of the airways and lungs contributed to by these organisms and irritants decreases the ability to breathe effectively.

Abesses
An abscess is caused by disruption of a normal tissue barrier through penetrating trauma, bites, established infections, or by migration of normal flora to other sterile areas of the body. It is an infection and inflammatory process; where by leukocytes (white blood cells which protect the body from invading organisms) form a wall around infectious agents in tissue, organs, or confined spaces, to localize inflammatory exudate (pus), in an attempt to prevent further spread of infection. Exudate (pus) is made up of dead leukocytes, dead organisms, and fluid from blood.

Most abcesses I have dealt with have further develop under the skin surface and appear visually as a pus filled lump. This pus filled lump may then "come to a head" by itself, rupture and drain, or through local application of heat, break through the skin and allow the pus to drain thereby resolving the abscess. They then require cleaning with some warm water and kept clean to allow the body to heal.

Signs of an Abesses:
•Abnormal swelling of tissue filled with pus.
•A swelling or lump that develops quickly from one to a few days. It may be located anywhere on the body and may cause tenderness.
•The center of the lump may appear white as the skin starts to thin signifying rupture, or may show a scab at the site of the lump where injury was initiated.
•If a deep abscess is present, or if there is systemic involvement, signs may include: anorexia, weight loss, possible limited mobility, and possible abnormal organ function.

Barbering
Barbering is a type of alopecia (hair loss) that is termed acquired, meaning that the rat was not born that way. It is believed to occur due to any one of the following reasons: either behavioral brought on by stress, boredom, over grooming by a more dominant rat over its cage mate, hereditary factors in which one of the parents, or grandparents exhibited signs. Siblings, as well, may or may not show signs of over grooming.
Because the rat chews the hair so close to the skin, it gives the appearance of being clean shaven, hence the term barbering. The most common places for barbering to be seen on the body is the stomach or front legs if over grooming self, or on the muzzle, head, or shoulders of a cage mate.

In barbering the condition of the skin itself is generally not affected and its appearance will be normal without signs of inflammation, irritation, or cuts. This condition, unless irritation should develop, does not require treatment.

Signs of an Barbering:
•Hair loss on abdomen, limbs, shoulders, head or muzzle, more often without signs of inflammation or scabbing (skin smooth and intact).

Mycoplasma / Mycoplasmosis
Clinical signs vary depending on virulence, and the site of infection. Also because the course of disease caused by mycoplasma is chronic rather than fulminant, signs of illness tend to increase as the rat ages. Any of the following clinical signs may be present:
In upper respiratory disease, signs may range from clinically silent to early signs of sneezing, snuffling, squinting, and porphyrin staining (rust colored) around eyes and nose. Inner ear infection may also be seen with signs of head tilt, rolling, and face or ear rubbing.
As disease advances along the respiratory passages causing bronchiolitis, bronchiectasis and bronchopneumonia, the signs may include rattling moist breath sounds, labored breathing, gasping, chattering, and coughing. Additional signs of illness are: hunched posturing with rough coat, weight loss, and changes in behavior due to illness (e.g. nipping, biting, avoidance).

In genital infections, the organism may be a cause of pyometra or purulent endometritis (inflammation of the lining of the uterus), salpingitis (inflammation of fallopian tubes), and perioophoritis (inflammation of ovaries). The signs may range from clinically inapparent symptoms to abdominal distention or signs of blood-tinged uterine discharge. Hematuria (blood tinged urine) from a concurrent urinary tract infection may also be present.
Where chronic uterine infections are attributed to Mycoplasma, decreased litter sizes may also result.

SDA is a highly contagious virus that weakens the immune system. Alone it is not fatal, however it opens the door for secondary bacterial infections that can kill. In a laboratory environment SDA has a low mortality rate. This is due to the fact that in the lab the secondary diseases are not in place to cause the infections that kill our rats.
SDA is an RNA coronavirus. It is an airborne virus that is contracted via respiratory aerosol commonly spread by direct contact with affected rats. Contact with food, bedding, cages, or other accessories used by infected rats can spread the disease. Viral secretions carried on human skin, clothing, or in mucus membranes can also contribute to the spread of this illness.

The secondary infections will persist for a longer amount of time, depending on the bacteria or other viruses involved.
In some cases the rats that survive the SDA outbreak have eye and/or respiratory damage for life.
Not all rats infected with SDA show symptoms.


Sendai Virus
Signs of Sendai:
•Sneezing
•Hunched posture
•Respiratory distress
•Labored breathing
•Porphyrin discharge from eyes and/or nose
•Lethargy
•Prolonged pregnancy
•Partial or full litter loss
•Failure to thrive in surviving babies and young rats
•Anorexia

Rats, mice, and hamsters are the exclusive natural hosts of Sendai virus (SV). The SV is immunosuppressive and can have an immediate, as well as a long term effect, on the rat’s immune system. SV infection may cause high morbidity (illness) and mortality rates when combined with bacterial pathogens such as mycoplasma pulmonis and CAR (cilia associated respiratory) bacillus. The secondary infection may also be caused by bacteria that are non pathogenic until the immune system is depressed, such as, pasteurella.

Infection is most devastating to very young, elderly, and immunocompromised rats who may develop a more severe pneumonia and in which the virus may persist for a longer amount of time. Athymic rats (rnu/rnu) are more susceptible to SV and can stay persistently infected.