Interesting article. There are hundreds of items in the average grocery store with 'may contain' on the label. Many manufacturers don't know exactly what is in their own product according to their own labels. It seems to defeat the purpose of labeling ingredients.
You want to know what's interesting? This article doesn't include the incidence of asthma in Africa.
And you want to know what else? It's lower than it is in the industrialized world--about the only disease that I can think of that is this way (outside of those induced by distinctly industrial phenomenoms). Funny thing is, low income African-Americans are the most susceptible in the US.
I've seen some evidence that suggests that the parasites that rural Africans are exposed to may decrease the incidence of asthma in the population, priming their immune system to avoid the IgE sensitivity you see with asthma.
Not really.
A food processor will run multiple batches of different produces on his machinery each day. If you process something with peanuts, wash down, then process something without nuts, it is probably legally safer say "may contain peanuts" or "processed on machines that were used to process peanuts" than to leave the warning off.