On average, the untreated HIV infected patient will progress to AIDS in about 5-10 years. Since 1996, the introduction of powerful anti-retroviral therapies has dramatically changed the natural progression of HIV infection to the development of AIDS.
You CAN feed your horse just hay, but you have to up the portions and it also depends upon the type of hay you are feeding. Alfalfa mix is normally the best but there again, you're goinng to have to feed more hay to make up for the not feeding grain when you are riding.
However, horses may also be fed small amounts of other grains like corn. Some grains like wheat, aren't good for horses. The seed head of grasses would be the closest thing a wild horse would come to eating grains in their natural environment. Grains that are grown, harvested and processed as we do now, are not natural foods for horses. It is easy to feed too much grain to horses. Grain also doesn't require the chewing time or contain the silica grass does and this can contribute to things like ulcers and dental problems. A horse that over-eats a large amount of grain may colic or founder.