Sure, high estrogen during your Week 2 can boost your mood, energy and confidence. But, there’s a downside to all this estrogen, too: At the end of Week 2 during ovulation (in the middle of your monthly cycle), your immune system weakens, making you more vulnerable to infections from bacteria, viruses and yeast, for instance, you’re more likely to catch a cold, be bothered by a stomach bug, develop an infection in a wound, catch an STD or struggle with a yeast infection. Researchers theorize that high estrogen triggers this drop in immunity to help prevent the body from attacking sperm during ovulation, upping your chance of pregnancy.

Luckily, you can take simple precautions when you’re entering this phase of your cycle to improve your chances of staying healthy, for instance, by washing your hands more frequently to avoid catching whatever bug is going around, cleaning cuts and sores, using condoms and reducing your intake of sugar, alcohol and refined carbohydrates, which can increase your risk for a yeast infection.