Insects don't have an adaptative immune system. That's why fungi are so lethal against them. You don't find such pervasive infections in immunocompetent humans.
Not true. There are some rather nasty tropical fungal species that cause persistent, tenacious infections even in healthy humans.
www.dermnetnz.org/fungal/systemic-mycoses.htmlThat's just one of three classifications of fungal infection. There are others-- these ones just happen to spread all through the body, which makes them more interesting from the discussion's point of view.
Besides, the argument against "competent scientists" is a misleading one. At the current rate of price reduction and equipment refinement of biotechnology solutions, it is only a matter of time before you can make super ebola in your kitchen, much like you can hack a cellphone on your weekend.
When that happens, the doors are opened to religious crazies, and general purpose nutters of all types. These include people who have simply decided to become powerfully misanthropic, and who's objective is not to conquer the world, but to end it. Think less government takeover, and more Unibomber-- with biologicals instead of bombs. Other contenders are religious extremists who believe their god will protect them in the ensuing plague, etc.
The real argument is that an engineered plague of this complexity would be unlikely to be made successfully on the very first try. It would likely require significant revisions and testing to properly tweak the biochemistry to be just right to cause the desired panopoly of symptoms. If you consider the religious extremist angle, then they are going to be either sacrificing themselves religiously for the work, or abducting "heathen sinners" for experimentation. In the case of Unibomber type crazy, you have run of the mill serial killer behavior, with death by zombification being the MO.
Either case could create a zombie apocolypse, if the right motivating circumstances and enabling features are present, but the lack of a very tightly confined and properly isolated/sterilized work area means that the resulting pathogen will likely not be "perfect", and is likely to escape before being "finished."