Women may be programmed to be more susceptible to infection during certain times of the month, according to new research published in the Journal of Leukocyte Biology.

Researchers from Spain and Austria found that women were most susceptible to sexually transmitted diseases or other infections during ovulation than any other time during the reproductive cycle. They theorize this adaptation may occur in order to allow male sperm to survive any potential immune system response in order to successfully fertilize an egg.

In their research with mice, those treated with the sex hormone, estradiol, had a lower immune response and were more susceptible a fungal infection.

“The next time you hear a woman say that she’s sick of men, you can add this to her list as another reason,” said John Wherry, Ph.D., deputy editor of the journal.