Suppose someone said, "X looks like someone with a venereal disease." That is known as a simile.
Now "X has a venereal disease" would constitute defamation (if untrue). But the statement was "X looks like..."
In the United States would the following, or other defenses hold: 1) "X looks like..." is a statement of opinion rather than fact, and therefore not defamatory?" 2) An expert (e.g medical) witness testifies that a picture of X looks like that of someone with a (certain) venereal disease, whether or not s/he actually has it?"
How about the United Kingdom, where defamation laws are more plaintiff-friendly?