Actually with the right technology, you can "infect" someone with a genetic disease. Typically it is done in reverse (curing) and as an embryo, but the process is the same. It would be more difficult after birth due to cell differentiation but not necessarily impossible. Of course this ignores the ethical questions, but would a Karlsson care?Seth and Astrid have is a rare genetic disease and it's absolutely certain that it's not an infectious disease, so you can't “infect” anyone with it, but you can inherit it.