1830s: After the death of Emperor Alexander, his brother Nicholas is made Czar of Russia. An armed uprising of young army officers and troops numbering nearly 3,000 men gathers in St. Petersburg to demand a republican government. Known as the Decembrist revolution, troops loyal to Nicholas quelled the uprising.

Today: The Russian political system is by no means stable. Communist radicals threaten to topple the democratically elected President, Boris Yeltsin. Economic problems add to the political instability.

1830: Cholera sweeps across Russia and spreads to the rest of Europe, killing millions of people. By the end of the year the disease kills 900,000 Russians.

Today: Cholera is very rare. Diagnosis and treatment for such airborne diseases has greatly improved, but the threat remains with the danger of germ warfare or a hybrid of existing contagions.

1830s: In Russia travelers use horses provided by post-stations to go from...