Yes, X-rays can be very harmful. X-rays are a form of electromagnetic radiation, which is also true of things like radio transmissions and light. However, the wavelength of X-rays is much shorter than light or radio waves (or to put it another way, the frequency is much higher), which means that each X-ray photon (a photon is like a packet of wave energy) carries enough energy to alter the DNA in your body’s cells if it strikes it in the wrong place. Another way of saying this is that X-rays are “ionising radiation”. If you’re exposed to a lot of X-ray radiation, it increases the probability of an X-ray photon causing a mutation in the DNA of a cell that could turn it into a cancer cell. If you got a REALLY high dose of X-rays, I suppose that would make you very sick by directly injuring a lot of your cells.
Too many x-ray can affect you but to die from them you need so much of them… People working in radiology for example, they have a gadget that they have always to bring with them; it always measure the x-ray radiation and measure the dose to which the technician has been exposed. When the dose is too much they need to stop working for few days.