The FP is an infectious disease with horizontal transmission. An alphaherpesvirus initially called fibropapilloma-associated turtle herpesvirus (FPTHV), and now called "Chelonid alphaherpesvirus 5", is believed to be the causative agent of the disease. The reason for this belief is because nearly all tissue samples tested from turtles with lesions carry genetic material of this herpesvirus, varying between 95 and 100% depending on different studies and locations. The DNA loads of the herpesvirus in tumour tissue are 2.5–4.5 logarithms higher than in uninfected tissue.