According to Did You Know.org:
Sharks are the only animals that almost never get sick: they are immune to almost every known disease.
Is this true or is it a myth?
Not only is this not true, but it is explicitly disproved by the sources provided by that very site. LOL!
Sharks do get cancer.
Further down on the same page as the claim, it says:
History of known shark tumors
Clicking on that link we get:
The promotion of crude shark cartilage extracts as a cure for cancer has contributed to at least two significant negative outcomes: a dramatic decline in shark populations and a diversion of patients from effective cancer treatments. An alleged lack of cancer in sharks constitutes a key justification for its use. Herein, both malignant and benign neoplasms of sharks and their relatives are described, including previously unreported cases from the Registry of Tumors in Lower Animals, and two sharks with two cancers each.
The claims that sharks do not, or rarely, get cancer was originally argued by I. William Lane in a book entitled “Sharks Don’t Get Cancer” in 1992 (7) , publicized in “60 Minutes” television segments in 1993, and reargued in another book in 1996 (8) . The titles of the books do not match their texts in which the authors note that sharks actually get cancer but claim incorrectly that sharks rarely get cancer. We make three main points below: (a) sharks do get cancer; (b) the rate of shark cancer is not known from present data; and (c) even if the incidence of shark cancer were low, cancer incidence is irrelevant to the use of crude extracts for cancer treatment.