Red-eye effects happen when camera flashes are triggered too close to the camera lens when shot in a low light. The bright light dispersed in a large amount that it passes through the eye pupil and bounce the light back out from the eyeball which is then recorded by the camera. The major reason as to why red eye occurs in flash photos is the generous amount of blood within the eye behind the retina that was flashed back to the camera.
The amount of redness shown within the eyes varies on the level of melanin stored behind the layers of the retina. The amount of melanin heavily impacts from person to person. For example, if we were to take a photo of a light-skinned person with blue eyes, they would have a major red-eye effect in photos as they have low production in melanin as compared to people with darker skin and eyes. The level of melanin also impacts animals who are also capable of displaying red-eyes depending on how much melanin they have stored behind their retinas.
Red-eye effects are easily prevented through different ways. Certain cameras have built-in red-eye reduction features that hold a few low blinking red light moments before the flash goes off. This feature will cause the pupil to contract thus reducing the amount of light from to the flash to bounce into the retina. Also, increasing the ambient lighting, reducing the shutter speed or increasing the lens aperture helps to take a clear picture without needing to rely on flash.
However, if there are still prominent red-eyes in your photos, there are free online picture editor red eye removal services that offers red eye picture removal that will help remove those red-eyes instantly without needing to manually edit those out yourself.