well, many cells n our body have a lifespan where they age & get replaced with newer ones "like hair" as long as the process is balanced where 1 aged cell get replaced by 1 new cell- everything works fine as it should b & no problems- but f the number of new cells outnumber the aged cells to b replaced- things get out of control & uncontrolled proliferation develops what we know as "cancer"- so the question would b what cause this ? it's caused by our body being malfunctioning- our body just like pc/mobile-apps it's made of a code that control our gender & our general appearance + it tells our body how to function & do the things that r "automatic by default" like our heartbeats, breathing, immune-system- that code is called "genes" & a faulty gene will cause our body to malfunction or not to function just like codes n pc/mobile-apps, a faulty gene is caused by a bad mutation- so the question would b what cause the bad mutation ? here ur answer comes "nutrition & toxicity- smoking, alcohol, unhealthy foods & drinks, pollution, stress, trauma, shocks, lack of sleep, lack of nutritional elements, low water intake""although some suggest that random bad mutation do happen" but hence genes r transmittable from the upper generation to the lower- a malfunction n the body of the new generation can b caused by a bad mutation that happened n the older generation "that can b ur direct ancestors or ur ancestors who lived centuries ago", unfortunately we didn't develop a better cure than radiotherapy & chemotherapy for cancer treatment yet they r used n younger patients, while for elders "usually 70+" they r given painkillers & "medicines" that reduce the symptoms for as many years as possible. this is how genes r transmitted across generations https://en.wikipedia.org/wiki/Mendelian_inheritance & this is an example of an inherited genetic disease https://en.wikipedia.org/wiki/Haemophilia & this is the incident of making the 1st genetically edited human to b HIV-resistant which happened n china https://en.wikipedia.org/wiki/He_Jiankui_affair . didn't plan to b that long- i hope that wasn't boring! haha. have a nice day