"Why is there a risk of disease between gay and lesbian people. What's up with that?"
It sounds like you are asking about risks of STDs for gay men and lesbians. Often we assume that gay men are the most likely to pass or contract an STD during sex. You might have even heard someone say, "Gay sex is more risky for STD's than straight sex." I know I've heard this, but I also know that this is not an accurate statement. In fact, anyone who has sex, is at risk. That is true whether you are gay or straight, male or female.
Still, certain sex acts do pose a greater risk for the transmission of certain infections than others. But none of these acts are limited to GLBT individuals.
According to the New York City Department of Health:
"Unprotected anal sex is the greatest sexual risk for spreading HIV. Receptive anal intercourse is 5 times riskier than receptive vaginal intercourse and 50 times riskier than receptive oral sex. Insertive anal or vaginal intercourse is 10 times riskier than insertive oral sex. Oral sex is less risky than other penetrative sexual activities, but is not without risk for both partners."
More than worrying about whether one person is more likely to transmit or contract an infection than another, a better bet would be to consistently practice safe sex and get tested for infections on a regular basis.