There are many reasons why Bad things happen. God gave us freewill; with that comes responsibilities along with the consequence of freewill. If you do bad things, bad things will happen to you or others.
Regarding illnesses that are outside our control; when God first created the world, He did not intend it to be the way it is now. It was supposed to be free of sickness and disease. After the 6 days of creation, God declared “it was very good” (Genesis 1:31 NKJV). Unfortunately, after the fall of man (ie Adam/Eve’s sin), everything went from bad to worse (and continues to do so) known as God’s judgement on sin because He is unable to tolerate sin (See this article).
People look at the world today (after the fall) and judge God’s handy work. When God originally made the world, it was totally different and faultless. There was supposed to be no death and no suffering including to all animals. The Bible says that God told Adam to be vegetarian: “And God said, ‘See, I have given you every herb that yields seed which is on the face of all the earth, and every tree whose fruit yields seed; to you it shall be for food.’” (Genesis 1:29) and “Also, to every beast of the earth, to every bird of the air, and to everything that creeps on the earth, in which there is life, I have given every green herb for food’; and it was so.” (Genesis 1:30). All this changed after the fall and after the Global Flood.
The good news is that God is trying to fix this world (which is what Jesus did when he died on the cross) so that it’s back as it was when he first created it. The Bible says that one day “The wolf also shall dwell with the lamb, and the leopard shall lie down with the kid; and the calf and the young lion and the fatling together; and a little child shall lead them” (Isaiah 11:6).
So we live in a sinful fallen world and Satan is running around deceiving and misleading people with lies. It is our choice to accept the free gift of God (ie accepting Jesus as our substitute for our sin) and follow the biblical teaching, or reject it. Our choices will have consequences in THIS life time (to us and others) and the NEXT (to us only).