What virus could be causing this? My 19-month-old boy was sick last week (fever for 3 days, peak fever at 39.4, cough). Now it's me.

There are several viruses that can cause the symptoms you mentioned, such as the rhinovirus that causes the common cold or even the influenza virus. It is important to rest at home and let the illness run its course. If your symptoms last longer than 3 days, it is recommended to seek medical advice. Additionally, it is possible to still have COVID-19 even if your son tested negative, so it is important to monitor your symptoms and seek help if needed. Stay safe and take care.