Statue of Lucius Annaeus Seneca the Younger in Cordoba, Spain, by Amadeo Ruiz Olmos
IX: On Philosophy and Friendship
These are the so-called “fair-weather” friendships; one who is chosen for the sake of utility will be satisfactory only so long as he is useful. Hence prosperous men are blockaded by troops of friends; but those who have failed stand amid vast loneliness, their friends fleeing from the very crisis which is to test their worth. Hence, also, we notice those many shameful cases of persons who, through fear, desert or betray. The beginning and the end cannot but harmonize. He who begins to be your friend because it pays will also cease because it pays.
Seneca Epistles 1-65, by Lucius Annaeus Seneca the Younger. Translation by Richard Gummere. Loeb Classical Library.
Prior to the advent of Covid-19, I was about 70% through the first draft of a novel about a virus, an anger virus. Looking back on what I wrote, I am stunned by how mild and self-contained the virus I created was. I am currently too busy panic cleaning, home schooling and hand washing to do much about the novel, but in the process of doing research into epidemiology and vaccines, I came across several books that helped me understand how viruses work, and thought now would be an excellent time to recommend them. 
Valerie Block 