Thanks @eudonova for sharing this information with us. I will let myself to paste here the Google translation I have found:
The purr is a characteristic sound made by all feline species and is part of its communication system. In domestic cats the purr has a frequency between 25 and 150 vibrations per second. This particular sound indicates confidence, happiness or well-being and occurs when they feel safe and protected. It is not known with certainty where this sound comes from, which rather constitutes a vibration. Different places have been attributed to it, without exactly locating it.

Several studies of recent years show that, indeed, the purring of the cat has many benefits for us, and acts on people in different planes, both physical, emotional and affective. These studies have shown that people who live with cats have fewer annual visits to the doctor, have fewer problems falling asleep, and are also less likely to take heart medication.

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