"Unhygienic environment" refers to a place that is not clean or sanitary, which can potentially lead to health problems or the spread of diseases. Full definition
In a bad scenario many kittens of different ages from different litters (and thus probably exposure to different viruses and organisms which will be shared with the other vulnerable kittens) are kept in crowded and unhygienic environments, without being vaccinated, and perhaps not getting the appropriate nutrition for kittens. (icatcare.org)
Clean bunny's house / tray daily - this not only stops any odours, but will ensure your bunny will be safe from fleas and other diseases which will be caused by an unhygienic environment (would you want to live in your own poo for days??). (boingonline.com)
Puppies bred in such facilities are more likely to have health related issues and genetic problems due to the inappropriate breeding habits inculcated by such breeders and the unhygienic environment of such facilities. (usbones.com)