Diseases can be spread across a wide spectrum, with predominantly genetic diseases at one extreme of the spectrum and diseases of largely environmental origin at the other. 

Between these two extremes lie most human diseases--those with both genetic and environmental causative influences that are significant. 

Indeed, even at the very extreme ends of the spectrum both factors play 'some' role.

The genetic constitution dictates in part the host's response to environmental challenges. 

Similarly, environmental factors play significant roles in the manifestation of genetically induced disease. 

Furthermore, there are many disorders in which there is a familial tendency to develop the disease 'but no formal pattern of inheritance has been delineated'. 

Many forms of cancer, high blood pressure, arthritis, and obesity, for example, 'seem' to have a familial tendency. 

Although the exact roles of environmental and genetic factors are 'unknown' in all these diseases, it is strongly felt that both factors contribute to the disease process.

Medical and Alternative Medical Terminology, Explanations -  by Courtesy of Encyclopedia Britannica® 2001

My questions are here : 

(1) What is the answer for the medical statement, mentioned above?

(2) Why (heredity) genetic diseases are not affecting all the members in a family?

(3) Why (heredity) genetic diseases are still (in this modern age) affecting only some - but not all of the members in a family?

(4) Why environmental conditions are not affecting all the
members in a same family?

(5) Why environmental conditions are still (in this modern age) affecting only some - but not all of the members in a family?

There must be some strong
reasons behind this.