When cancer becomes contagious: Three cases that defy medical logic

Ordinarily, cancers are not regarded as contagious — whereas AIDS, COVID-19, tuberculosis, and malaria are. Contagious diseases spread from one person to another through direct contact, airborne droplets or through insect vectors like mosquitoes. On the other hand, cancers originate from one of the patient’s own cells.

However, so far, scientists have documented three instances in which naturally occurring cancers in animals became contagious. That is, the cancer cells originated in one individual, then moved into another, formed tumours in its new host, and then moved on in search of still other hosts.