We have to balance these cases with the huge number of people who are exposed to high levels of Aspergillus spores as part of their occupations - in the main they are farmers working with grain and hay. These people do not seem to be at risk of death as few cases are reported - though of course some may not be diagnosed.
We discussed one way a person with a normal immune system may become overwhelmed last week.
The likelihood is that each person that has died has an important, possibly quite specific defect in their immune system that leaves them vulnerable to attack. Age may be another factor as we do know the elderly have a less efficient immune system compared with younger adults but it certainly isn't the only factor. The National Aspergillosis Centre in the UK and others are carrying out research into what makes people vulnerable to infection by Aspergillus (see talk by Paul Bowyer, Principle Scientist) and seem to have made significant progress in considering multiple possible genetic pathways but there is much more to do.
Regardless it is currently impossible to identify individuals vulnerable to being overwhelmed by Aspergillus, so it is important we work to maximise awareness when carrying our risky tasks in the vicinity of moulds and/or working with hazardous materials e.g. turning over compost, opening bags of compost, opening bags and spreading bark chippings and more.