Complex sentence: On the night before the marriage, Lingesan comes to know from another doctor that he is not suffering from an incurable genetic disease but from a disease called H4N2 influenza, caused by the "I" virus, which can only be transmitted by injection.
Split into simpler sentences: On the night before the marriage, Lingesan comes to know from another doctor that he is not suffering from an incurable genetic disease, but is suffering from a disease called H4N2 influenza. The virus which causes the disease is called the "I" virus and can only be transmitted by injection.