It is spread human to human via liquid droplets from coughing or sneezing. Attaches to ACE-2 receptors in the lungs and then duplicates inside the affected cells, destroys the tissue and attacks the immune system. It can be killed inside the body by lymphocytes (killer cells, T cells) which detect the infected cells and then attack them by injecting them with enzymes that destroy and dissolve them. So keeping your immune system frosty during this time is a good idea. (less alcohol, no smoking, plenty of water, antioxidants, vitamins/minerals, helpful herbs, and a well-balanced diet if possible.
From ice core samples, (measurements which were made prior to 1950), the levels have been between 250–300 ppm for thousands of years (quite stable). (There are some controversies over the accuracy of using ice core samples). So the general consensus backed by evidence would show that CO2 levels have increased sharply during the last few hundred years. So if we take 250ppm increasing to 400ppm we get 37.5% increase (or what is commonly heard, 40% increase over the last several hundred years).