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Effects on human health
Because of their small size, domesticated house cats pose little physical danger to adult humans. However, in the USA cats inflict about 400,000 bites per year. This number represents about one in ten of all animal bites. Cat bites may become infected, sometimes with serious consequences such as cat-scratch disease, or, very rarely, rabies. Cats may also pose a danger to pregnant women and immunosuppressed individuals, since their feces, in rare cases, can transmit toxoplasmosis. A large percentage of cats are infected with this parasite, with infection rates ranging from around 40 to 60% in both domestic and stray cats worldwide. Research indicates a correlation between the parasite Toxoplasma gondii, which sexually reproduces exclusively in cats, and numerous human psychiatric conditions, including OCD. The compulsive hoarding of cats, a symptom of obsessive compulsive disorder (OCD), has long been associated with "crazy cat ladies".

Allergic reactions to cats are relatively common, happening in as many as every 3 in 10 Americans. The major allergen, Fel d 1, is found in the saliva and/or dander of all cat breeds. There have been attempts to breed hypoallergenic cats, which would be less likely to provoke an allergic reaction but. Some humans who are allergic to cats—typically manifested by hay fever, asthma, or a skin rash—quickly acclimate themselves to a particular animal and live comfortably in the same house with it, while retaining an allergy to cats in general. Whether the risk of developing allergic diseases such as asthma is increased or decreased by cat ownership is uncertain. Some owners cope with this problem by taking allergy medicine, along with bathing their cats frequently, since weekly bathing will reduce the amount of dander shed by a cat.

As well as posing health risks, interactions with cats may improve health and reduce physical responses to stress: for example the presence of cats may moderately decrease blood pressure. Cat ownership may also improve psychological health by providing emotional support and dispelling feelings of depression, anxiety and loneliness. Their ability to provide companionship and friendship are common reasons given for owning a cat.

From another point of view, cats are thought to be able to improve the general mood of their owners by alleviating negative attitudes. According to a Swiss study carried out in 2003, cats may change the overall psychological state of their owner as their company's effect appears to be comparable to that of a human partner. The researchers concluded that, while cats were not shown to promote positive moods, they do alleviate negative ones.

The purring from cats is also believed to contain healing properties. An individual cat's purring can range from 20 Hz up to 150 Hz. These vibration levels have been proven to aid in bone growth and repair. A year-long study done by Dr. Clinton Rubin and his team found that bones of older sheep began showing signs of repair after being exposed to 30 Hz of vibrations for 20 minutes, five days a week. For humans, vibrational therapy has been found to not only aid in bone repair but also strengthen tendons and joints. Cats purrs are believed to work similarly on the human body alongside reducing risk of blood pressure, heart disease, and symptoms of dyspnea.

One study found that cat ownership is associated with a reduced risk of heart attacks and strokes at the 95% confidence interval.

Several studies have shown that cats develop affection towards their owners. However, the effect of these pets on human health is closely related to the time and effort the cat owner is able to invest in it, in terms of bonding and playing.

Therapy cats
Some cats, called "therapy cats" are trained to help ailing humans in a medically beneficial way to take advantage of the human-animal interaction for purposes of relaxation and healing. Certain breeds are desirable when looking into therapy cats due to their personality and temperament. Some examples of preferred breeds are Ragdolls, Main Coons, American Shorthairs, Siamese, and Persians. The important traits to look for in a cat include a steady demeanor with tolerance to sights and sounds that are unfamiliar or sudden, petting that could be awkward or rough, and the ability to stay calm when being poked or pulled in unusual manners. Therapy cats must also be acclimated to humans of all ages and enjoy engaging with strangers daily. Some therapy cats are used as alternatives to therapy dogs due to the cats size and nature, allowing them to work with patients/people who might otherwise be scared of dogs. Therapy cats should still be accustomed to dogs since most visits happen in conjunction with one another.

The presence of cats in addition to their purring as well as petting them can deliver both psychological and physical benefits. Therapy cats are being used as companions to help the recovery and well-being of people who have had strokes, high blood pressure, anxiety,  and/or depression to name a few.

Therapy cats are utilized as companions at juvenile detention centers; for children with developmental disabilities; and for children with language, speech and hearing difficulties. Therapy cats are also sometimes used in hospitals to relax children who are staying there, as well as helping those in hospice care cope with their terminal illness.