Some viruses appear to vanish when outbreaks dissipate, only to emerge again. Scientists don't always know which animals they lurk in — and how to stop them from spilling back over. Everything comes from somewhere.

Dangerous new viruses typically come from animals. Human influenza viruses come from wild aquatic birds. Hantaviruses come from rodents.

Mpox virus also comes from rodents, including squirrels. The pandemic-causing form of H. I.

V. came from chimpanzees. Viruses can replicate only in the cells of cellular creatures, making animal viruses most likely to adapt to humans. Identifying the animal source allows scientists to infer how and where transfer occurred, guiding prevention measures.

In 1998, Nipah virus began killing people in Malaysia, traced to fruit bats attracted to orchards above hog lots. Deforestation drove bats to orchards, leading to pig infections and human spillover. Separating orchards from hog lots helped prevent future outbreaks.

For ebolaviruses, common knowledge says they come from bats, but evidence is lacking. Ebola has caused dozens of outbreaks in Central and West Africa, with over 7,400 cases in the current DRC outbreak. The kind of animal in which a virus abides is known as the reservoir host.

When the virus passes into a human, it's called spillover. These concepts are crucial for outbreak control, though the most important question for prevention remains unanswered.