When it comes to your health, with my Garmin CIQ HeartMonitor+HRV it is easy to measure and track all kinds of information. Have you ever wondered what the health impact of a stressful day was? Will you perform well during your long run tomorrow morning? Is there anything you can do today that would improve your ability to have a better day moving forward? Are you about to get sick (including COVID-19)? HRV is the piece of data that could help you answer these questions.
What is HRV?
HRV is simply a measure of the variation in time between each heartbeat. This variation is controlled by the nervous system called the autonomic nervous system. The Autonomic Nervous System (ANS) is always working and regulates, our heart rate, blood pressure, breathing, and digestion. The ANS is subdivided into two components, the sympathetic (fight-or-flight mechanism) and the para-sympathetic (relaxation response) nervous systems.
Our brain is always processing information in the hypothalamus. The hypothalamus, through the ANS, sends signals to the rest of the body either to stimulate or to relax different functions. It responds not only to a poor night of sleep, or that tough day, but also to the exciting news that you got engaged, or to that delicious healthy meal you had for lunch. If we have persistent stresses as illness, poor sleep, unhealthy diet, dysfunctional relationships, isolation or solitude, and lack of exercise, this balance is disrupted, and your fight-or-flight response can shift into overdrive.
Why check Heart Rate Variability?
HRV is noninvasive way to identify the ANS imbalances. If your nervous system is in a fight-or-flight mode, the variation between subsequent heartbeats is low. If your nervous system a more relaxed state, the variation between beats is high. In other words, the healthier the ANS the faster you are able to switch gears, showing more resilience and flexibility.
People who have a high HRV may have greater cardiovascular fitness and be more resilient to stress. HRV may also provide personal feedback about your lifestyle and help motivate those who are considering taking steps toward a healthier life. It is fascinating to see how HRV changes as you incorporate more https://www.health.harvard.edu/heart-health/mindfulness-can-improve-heart-health, meditation, sleep, and especially physical activity into your life.
For those who love data and numbers, this can be a nice way to track how your nervous system is reacting not only to the environment, but also to your emotions, thoughts, and feelings.
How can Heart Rate Variability detect illness?
HRV is good for detecting illness (Colds, Viruses including Coronavirus disease (COVID-19)) because it is sensitive to how stressed the body is at that moment. HRV directly taps into the balance between the stress (fight or flight) sympathetic and the parasympathetic (rest or digest) branches of the nervous system.
Lower HRV indicates that the body is more stressed. Although you might expect that different kinds of stress e.g. physical, mental, nutritional, affect your body differently, in fact at the base level they affect the nervous system in very similar ways. So, if you are mentally or emotionally stressed and you eat poorly (e.g. seeking comfort foods) your ability to handle training stress is much reduced.  There is an entire series here on managing https://www.trainingpeaks.com/coach-blog/the-whole-picture-an-introduction-to-total-load/.
Detecting the start of illness
The example below shows a typical HRV chart for a recreational athlete with a low resting heart rate between 50-60bpm (red dashed line), and a high level of HRV ~100 (blue baseline). The colored dots are the daily HRV indications i.e. Green = go, Orange = caution, Red = stop. 
Using HeartMonitor+HRV, monitor ‘Average HRV score’ and ‘HRV +/-‘ values. Sudden changes in these two values is an indication of stress to your ANS. This stress could be due to hard workout, Daily Stress, Diet, On-coming sickness/illness
Creating a simple table to record your daily HRV and HRV +/- to monitor your health issue all you need to do.
More information pertaining to HRV and illness detection.