WASHINGTON - The AIDS virus has hideouts deep in the immune system that today’s drugs can’t reach. Now scientists finally have discovered how HIV builds one of those fortresses — and they’re exploring whether a drug already used to fight a parasite in developing countries just might hold a key to break in. Researchers have long struggled unsuccessfully to attack what they call reservoirs of dormant HIV, and the new work is in very early stages. But University of Rochester scientists say it may be fairly straightforward to attack one of these reservoirs, blood cells called macrophages that HIV hijacks and turns into viral hideaways.