Masked family (1918)

The 1918 Spanish flu was significantly deadlier than COVID-19, both in absolute and relative terms, though the two are roughly comparable in raw US death counts. 

The US population in 1918 was roughly one-third of today's, so the Spanish flu's percapita impact was about 4× greater than COVID-19's. The 1918 flu uniquely killed healthy young adults at high rates (8–10% mortality in that age group), while COVID-19 has disproportionately killed the elderly. In terms of cumulative years of life lost, the 1918 flu's impact remains greater.

Dogs can get canine influenza (CIV), a true influenza caused by two type A viruses:

• H3N8 – originated from horses, first identified in 2004 in Florida greyhounds. Not actively circulating since 2016.

• H3N2 – originated from birds, first appeared in U.S. dogs in 2015 in Chicago. Still causes localized outbreaks, often seeded from Asia.

Dogs can't catch human flu and can't give it to people. About 10–20% of infected dogs develop severe pneumonia. A vaccine is available.

Cats have two separate situations:

• "Cat flu" (the common term) is actually a feline upper respiratory infection caused by feline herpesvirus (FHV-1) and feline calicivirus (FCV) — not a real influenza virus. It's the feline equivalent of a cold, not the flu.

• Cats can be infected with actual influenza viruses (H1N1, H3N2, H7N2, H5N1, etc.), but there's no endemic feline influenza. The H3N2 canine flu has been confirmed to spread cat-to-cat in shelters. Cats are particularly vulnerable to severe, often fatal illness from H5N1 avian influenza, with dozens of cases reported since the 2024 U.S. dairy cattle outbreak. 

• In short: dogs get their own version of the flu; cats mostly get a "cold-like" respiratory infection, but can also pick up real influenza strains — especially bird flu — which can be serious.