The basic idea here is that conserving genetic diversity among the plants and animals of the world may allow scientists to someday use those plants and animals to find or create drugs that will be useful for the treatment of human diseases.
There are already many drugs that are in use today that come from plants and animals. As one exotic example, there are drugs that are made from the venom of poisonous snakes. There are two drugs that were based on snake venom and are used on people who may be having heart attacks or may be at risk of heart attacks.
If we lose biodiversity, we lose the chance to find other such drugs. When species become extinct, there is no longer any chance that any of their attributes will ever be used to create things that will be beneficial to human beings. Snake venom, for example, is being tested for eventual use in treating Alzheimer’s. If we allow snake species to go extinct, we will never know if their venom could have been used to combat one of the worst of human diseases.
This is the connection between conservation genetics and human diseases. Conserving wild species may someday allow us to use products from those species to combat diseases that harm us.