Do you mean fear of what disaster science could cause in the world? Or fear that will happen within our personal research?
The advances of science and technology could overtake our capacity to control the consequences. Synthetic biology, where we are creating artificial life forms for agriculture and medicine could have unforeseen ecological side-effects. Also, it could be argued that with increasing lifespan comes an increasing population which we do not have the resources or food to sustain. Nanotechnology, working at a molecular or atomic level, could also become highly destructive if used for warfare.
My worst fear for my own research seems very minor in comparison but it’s probably that I find out that the cells and DNA I have been working on have had some huge contamination which means that all my work from the last three years means nothing! (Let’s hope that’s not the case!)
My worst fear about science going wrong would be some researcher out there accidentally created a highly infectious strain of incurable disease that takes a long time to show symptoms (for example, AIDS that can be passed on through air, don’t think that exist at the moment). The worst intentional science fear would be biological/chemical warfare.
My worse fear about my own work would be making a mistake in the interpretation/preparation of my data and have an inaccurate conclusion that negatively impacts the health of others.
Antibiotic resistance! Bacteria have long been the leading cause of death in humans and before antibiotics this was much worse. We’re now facing a scenario in which there is widespread resistance in a number of bacteria to antibiotics. The problem is antibiotics are typically massive complicated molecules and it has proved incredibly difficult to invent new ones!
As such, much more needs to be done otherwise diseases such as the plague and tuberculosis could once again kill millions of people.
Unfortunately this doesn’t need to be imagined as in parts of the world with less access to medicines than the UK this is already the case as a result of various bacterial infections which are easily treated here.