How do illegal immigrants actually harm?

Our society maintains an infrastructure of support to its citizens, paid by its citizens, earned by its citizens and defined by them. Whenever a non-citizen makes use of the infrastructure resources, it reduces the availability to those who rightfully deserve it.
Roads, water supply, sewers, power grids, telecommunications, social services, markets etc. are examples.
Your family is limited in its resources...how would an uninvited stranger living in your home, eating your food, using your electricity etc. affect your life? There is some degree of degradation to you in every aspect of your family's life by the presence of that stranger.
The degree of degradation of infrastructure resources, and the increased cost of the infrastructure, is significant in direct proportion to the number of illegal immigrants. 11-12 million illegals living within the 300 million persons in the United States may only be 3-4 percent nationally, but in some border areas that percentage is much much higher. Additionally the great majority of these require above average infrastructure support and contribute below average support for the infrastructure...medical care for example...making the effect more like 8-10 percent.

So take a 10% cut in your income and consider the impact.