Note: My sister works for one of the largest clinical development, testing, and commercialization companies so I know some of the key issues but not all. This answer does not constitute advice on any particular stock or other instrument. This is mostly well researched opinion.
The problem with biotech companies (and a few other areas of technology) is that a lot of money is spent, and debt incurred, on ensuring that products are effective and safe to go to market. At any stage these tests can fail and the product is essentially worthless. At this stage the developers will have learnt a lot about the drug and how it is as efficacious as it is and so the next iteration of the potential drug will be better and hopefully less likely to cause complications and harmful side effects. The process of gaining approval for this second iteration is just as expensive, if not more so, than the last. This means that they are spending a lot of money on the drug and, for small biotech companies concentrating on one or few drugs, will have little to no income generation to offset this. If the money runs out before they get the product out they are bankrupt even if the drug is perfect.
A second issue is that they are not the only firm looking for a cure. They might have a very good drug that works very well but another company may have a better one in the pipeline that will either take their monopoly position or take all of their business based on the relative cost and efficacy. The longer it takes them to get through testing, the more likely it is that this will happen and the more likely it is that the competing drug will be first to market and receive all of the free publicity that goes with that. In this case the risk is that they have a product (eventually) but no market for it and so will again run out of money.
Another consideration is what the cure is actually worth. Prevention and awareness is already reducing the number of (wealthy) western people who have HIV and so the market size is falling where the most profit can be made. In order to get any return on your investment a profit will be required. Where HIV rates are rising is in poor countries in Africa, Asia, and south America where the price at which people could afford to buy a cure is likely to be lower than even the break even price for the firm. In this case you have a monopoly and a drug that works but no one can afford to buy it for a price that you can accept and still make a profit.
Biotech is a very risky, but potentially lucrative, area because there are just so many risks at every stage. Price volatility occurs on rumour and questionable statements from the company (who are always trying to be positive so that their funding doesn't dry up) and even relatively small trades can move the market a large amount as few people want to sell an investment with so much potential.
There are also some charged political positions with regard to HIV and AIDS, so a shift in political power could also derail a biotech firm that is researching this kind of drug.