Hataraku Saibou / Cells at Work! - Enthusiasm

Poor Red Blood Cell… he was so excited to work in a capacity that would bring about new life, only to be told after the fact that only a very, very small percentage of sexual acts actually result in procreation.

And it turns out that the host wasn't practicing safe sex and got gonorrhea, resulting in a shit ton of White Blood Cells getting killed. Way to kick a cell when he's down.

It's from Black, the male red blood cell with glasses is the protagonist.