My HormonologyOnce the second half of your Week 2 rolls around (approximately 10 to 14 days after the onset of your period), start taking more precautions to protect yourself from germs, for instance, by washing your hands more frequently, carrying around antibacterial gel and using a paper towel when touching grimy handles on public bathroom doors. During this phase of your cycle, your mucus thins and your immune system weakens, making you more vulnerable to catching whatever virus is going around. Why does this happen? Researchers explain it’s all about helping your body conceive during ovulation: Thinner cervical mucus allows sperm to enter the cervix more easily and a weaker immune system prevents your body from attacking the sperm.