Complex sentence: Some viruses, known as oncoviruses, contribute to certain forms of cancer; the best studied example is the association between Human papillomavirus and cervical cancer: it is now acknowledged that almost all cases of cervical cancer are caused by certain strains of this sexually transmitted virus.
Split into simpler sentences: Some viruses, known as oncoviruses, contribute to certain forms of cancer. The best studied example is the association between Human papillomavirus and cervical cancer: it is now acknowledged that almost all cases of cervical cancer are caused by certain strains of this sexually transmitted virus.