Cancer is the obvious start. An early concern with mobile technology was clusters of the disease around those living near phone masts. One study in Israel found a 4.5-fold increase in cancers of all kinds in the immediate vicinity of a mast (Int. J. Cancer Prev., 2004). In 2009, a Korean team of researchers carried out a pool analysis of the results of 23 studies, which involved almost 38,000 subjects.  
As with exposure to ‘anything’ sun, pollen, pharmaceuticals, there will be a certain percent of population that is ‘sensitive or hypersensitive’ to wifi / emf frequency. I am definitely sensitive with my husband doing his own control experiment by holding his wifi phone behind by head without my knowledge and me In pain from a piercing wave going through my head. Why is it that it must be all or nothing. There are those who will never be sick from wifi/ emf and those of us who live with it everyday.
There was no significant difference between sperm counts and sperm morphology excluding sperm motility, due to mobile phone usage period, (p = 0.074, p = 0.909, and p = 0.05, respectively). The total motile sperm count and the progressive motile sperm count decreased due to the increase of internet usage (p = 0.032 and p = 0.033, respectively). In line with the total motile sperm count, progressive motile sperm count also decreased with wireless internet usage compared with the wired internet connection usage (p = 0.009 and p = 0.018, respectively). There was a negative correlation between wireless internet usage duration and the total sperm count (r = −0.089, p = 0.039). 
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WiFi operates in the 2 to 5 GHz range -- part of the microwave portion of the electromagnetic spectrum. This is in the same part of the spectrum where cell phones operate so I may refer to WiFi or cellphone electromagnetic radiation interchangeably. These are radio waves -- no different from those used to broadcast television programs, except that they are higher in frequency. They aren't nearly as high a frequency as visible light, and no one worries about getting cancer from visible light (ultraviolet light, on the other hand, causes skin cancer, but this is the minimum energy necessary to cause ionizations that can cause breaks in strands of DNA, which is the mechanism by which cancer cells can be created). There is no credible evidence that non-ionizing radiation has any adverse health effects at all. There is no radiobiologic mechanism that could explain such an association -- and absolutely no scientifically valid evidence that this has ever happened.
It is useful to be aware of new health research regarding cell phone usage and cell phone radiation. The first cell phone call was made in 1985 and that phone cost $5,000 and weighed about 9 pounds. The change in size, weight and cost of devices today has probably led to over 50% of the human race owning a mobile device, the fastest growing technology on the planet.