The government launched a campaign to reduce the risk of doctors, nurses, lab technicians and other medical workers contracting AIDS and hepatitis, both viral diseases transmitted in blood. The Labor and Health and Human Services departments issued guidelines urging that all blood and body fluids be handled as potentially infectious. The federal Centers for Disease Control estimates that as many as 18,000 health care workers may be infected by the hepatitis B virus. A vaccine to prevent hepatitis B has been available since 1981 and a better one went on the market last year. But there is no way to safeguard against AIDS other than avoiding exposure to the HIV virus that causes it.