Breath that causes disease
Though the source says "venomous," you do not specify the "disease."
What about breath that causes vomiting? If it smells like human vomit that does cause a lot of people to throw up in real life. It could also have elements of disgustingness that would lead to even more people heading for the buckets.
If you'd like a more traditional disease, imagine Zoonotic influenza. We already have diseases like this:
Influenza A viruses infect humans and many different animals. The
emergence of a new and very different influenza A virus with the
ability [sic] infect people and have sustained human to human transmission,
can cause an influenza pandemic...Aquatic birds are the primary
natural reservoir for most subtypes of influenza A viruses. Most cause
asymptomatic or mild infection in birds, where the range of symptoms
depends on the virus properties.
It's not a stretch to imagine that an aquatic flying reptile has similar physiology to an aquatic bird. So imagine a flu that may or may not cause symptoms in a guivre. The guivres recover and stay carriers. The flu is airborne (though direct contact with the guivre's breath is needed in most cases) and can cross species lines and infect humans. How bad the infection is is up to you.
You can also combine these things. A big wham of guivre breath in a human's face causes immediate vomiting and then flu symptoms a few days later.
Fear of naked humans
The naked human bit (not in your source) would have to be psychological/behavioral.
Perhaps over time the guivre has learned that humans with coverings over their faces are safe to be around (it's protection from the guivre's breath; the humans know it protects against the smell at least). A human without a face covering leads to punishment (humans who suddenly vomit are not kind, and they cause people to yell a lot and probably beat the guivres).
Guivres are smart dragons but not quite cognitively advanced enough to understand the difference between covering the face and covering the body and why it matters. Nakedness scares them because they associate cloth on humans with calmness and kindness and absence of cloth with screaming and throwing things.
The humans aren't bright enough either to understand that the sudden vomiting isn't part of the disease that comes a few days later. Rather, they think it's the first symptom (and they aren't wrong from a certain standpoint, just a biological one). So if a guivre causes someone to vomit, this is very very scary and they react accordingly. The guivres in turn are terrified.