Poison Gas Why Diagnosis is Difficult (article) They could fake pandemic

Is it possible to fake a real pandemic (in that people get real symptoms of severe sickness) The people get bizarre flu like symptoms that they call sars, mers cov or H7n9 but is actually nerve gas chem agent poisoning ingestion or exposure.sprayed or put in the water?

First Link Poison Gas Attacks : Why Diagnosis Is So Difficult
Needs to be read in full but a couple of paragraphs stand out
“More over the diagnosis is even more difficult to make if a doctor first examines such a patient many days after onset of symptoms and after healing has obscured key features”
Ïn humans the chemical traces of the mustard gas disappear faster than the wounds heals”(Dr J Ambre)

“Laboratory tests to detect evidence of mustard gas in humans require expertise and sophisticated equipment that are available only in a few laboratories in the world”(Col M Dunn)

The other Link Manifestations of Toxic Effects
“Because the body only has a certain number of responses to chemical and biological stressors, it is a complicated business sorting out the signs and symptoms and determining the actual cause of human disease or illness. In many cases it is impossible to determine whether the illness was caused by chemical exposure or by biological agent (like the flu virus)

I first came up with this theory through listening to speech reversals. Would this be too difficult to pull off? If so why.