To infect a person and cause disease, viruses need to get inside individual human cells and reproduce. After a virus particle (called a virion) overcomes the cell’s defenses and enters the cell’s control center, it hijacks cellular machinery to churn out copies of itself. The copied virions burst out and invade other cells, spreading the infection. The new coronavirus that’s causing the global pandemic, called SARS-CoV-2 (the bristly-looking ball in the middle of the image), targets pneumocytes, the respiratory cells lining air sacs in the lungs. To gain entry, SARS-CoV-2 uses a molecular “key” called a spike glycoprotein (the triangle-shaped protrusions from the virion’s surface) to open a pathway into the pneumocyte’s interior. SARS-CoV-2’s spike proteins — its keys — fit a specific “lock” (technically known as a receptor, and in this case, the ACE2 receptor) on a pneumocyte cell’s surface. The ACE2 receptors are the small pink nubs just below the floating SARS-CoV-2 ball. When lock and key (spike protein and ACE2 receptor) connect, the virion gains entry and begins its destructive work. Scientists recently figured out the shape of SARS-CoV-2’s key and the ACE2 lock on respiratory cells’ surfaces. That knowledge could lead to treatments for infected patients, or a vaccine that would block infection.