My apologies, but Stryder, what the hell are you talking about?
With every disease that your ancestors have had, a preportion of your genetic code has been keyed to dealing with those particular diseases.
Immunity is not inherited. We gain immunity through vaccinations to the more serious infections that we face, like you mentioned, but we are forced to become immune to lesser pathongenic organisms through exposure to them. If I am inoculated for, say tetanus, to continue with your example, it does not mean that my children will be immune to tetanus infection.
Another example is the chicken pox. This is caused by a virus. When you are infected the first time, you develop red marks as a symptom. However, your immune system soon recognizes the viral particles as foreign, and creates antibodies against them (specifically against surface proteins on the viral coat). Since your immune system will continue to produce these antibodies, you will be protected from future infection. However, children of people who have become immune to chicken pox still must go through the pain-in-the-ass that is infection before they become immune.
What I noticed is if those disease although defunct exist within your genes, certain forms of radiation could quite easily retrigger some disease that you originally didn't have.
Do you have a link with a source for this? I haven't heard of this happening before, and it is quite unlikely.
Take for instance most people can only have Chicken pox once, now if your ancestors all had chicken pox once, it most definitely means if you haven't had it, you will eventually get it with or without somebody else making you contract it.
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Eman Resu:
Cancer is caused by mutations in the DNA that affect the cell's division cycle. There are numerous ways this can happen. If certain proteins are over-expressed, or others are under-expressed, then the cell can begin to divide without control, and a tumour will develop. Cancer cells are immortal, in that they don't have a limit to how many times they can divide, which also leads to problems.
Viruses are a different problem altogether. They are foreign particles (basically, they are a nucleic acid, either single-stranded or double-stranded RNA, or single-stranded or double-stranded DNA that codes for a few genes, and a protein coat) that enter the cells and try to use the hosts replication and transcription 'machinery' to make copies of themselves. In order to enter the cells, they need to have recognition proteins on their surface that bind to receptors on the cell they are trying to enter. These surface proteins on the viral coat are recognised as foreign by cells of the immune system (becoming antigens), which create antibodies against them. These antibodies bind to the surface proteins on the virus, and act as 'flags' that tag the particle for destruction by the immune system.
Diabetes is again different. It can be caused by a mutation, or it can be inherited. Depending on the type of diabetes, there is either too much, or too little insulin produced by the liver, which leads to too little, or too much glucose in the blood respectively. If you disrupt the pathway that leads to the creation of insulin in any way, then you decrease the amount of functional insulin, and increase the blood-sugar level. However, if a mutation causes an inhibitor of insulin to be damaged, then the insulin level becomes too high, and blood-sugar levels drop.
I don't know enough about heart disease to comment.