Viruses insert their genomes into our cells quite often.
You may have heard of cervical cancer and Human Papilloma Virus (HPV); cervical cancer can be the result of HPV inserting itself into the cervical cell’s genomes.
HPV is believed to cause cancer both by integrating into DNA and in non-integrated episomes.[40] Some of the “early genes” carried by the HPV virus, such as genes E6 and E7, act as oncogenes that promote tumor growth and malignant transformation. Furthermore, HPV can induce a tumorigenic process through integration into a host genome which is associated with alterations in DNA copy number.[41]
Also, we have experimentally observed ERV insertions occuring