My son was asking me how you know that all eggs are kosher. Maybe the hen it came from had an inoculation or something that punctured an organ or maybe the hen was dropped.
The reason why eggs are kosher, in spite of the concern that perhaps the hen was dropped etc., is an assumption – a chazakah – that animals are generally not treif.
The Gemara writes that the majority of animals are not treifos, and this “majority,” which is in fact a natural phenomenon (known as a ruva deleisa kaman), forms an assumption whereby any given hen is kosher unless we have reason to believe otherwise.
Based on this assumption it follows that eggs are likewise assumed kosher unless there is concrete reason to believe otherwise.