Examples of poor logic.

Many people assume that any personal attack is an ad hominem argument, but that isn't true. Some attacks aren't ad hominem fallacies, and some ad hominem fallacies aren't clear insults.

What the concept argument​ ad hominem means is "argument against the man." I.e. instead of criticising what a person says and the arguments they are offering, what we have instead is a criticism of where the arguments are coming from (the person). This is not necessarily relevant to the validity of what is said — thus, it is a Fallacy of Relevance.

The general form this argument takes is:

1. There is something objectionable about person X. Therefore, person X's claim is false.

Valid ad hominem arguments
It is important, however, to remember that an argumentum ad hominem is not always a fallacy! Not everything about a person is irrelevant to every possible topic or any possible argument that they might make. Sometimes it is entirely legitimate to bring up a person's expertise in some subject as a reason to be skeptical, and perhaps even dismissive, of their opinions about it.

For example:

1. Niddy is not a biologist and has no training in biology. Therefore, her opinions about what is or is not common with regards to human biology do not have a lot of credibility.
The above argument rests upon the assumption that, if a person is going to make credible assertions about what is or is not common for human biology, then they really should have some degree of intelligence enough to understand current biological knowledge — if not a training in biology such as a degree and perhaps some practical experience.

Now, to be fair pointing out the lack of intelligence or knowledge does not qualify as an automatic reason for declaring their opinion to be false. But when contrasted with the conclusions offered by a person who does have higher comprehension skills — or has relevant training, experience and academic knowledge — then we have a sound basis for not accepting the first person's statements.

This type of valid ad hominem argument is therefore valid.