And killed very few people. That's what we call terrorism (lots of news coverage for very few deaths) - it's not effective in doing anything but news cycles.
That's literally the desired effect. What you're thiking of is war.
> It sounds like you could infect thousands/millions
Its not the right way to infect millions of people. It does not scale. Therefore not effective.
"The unlawful use of violence and intimidation, especially against civilians, in the pursuit of political aims" is a definition of terrorism. Death count is irrelevant (outside of spreading fear itself). Zero deaths in a terrorist attack could still be a very effective attack. Spreading something that causes a hideous disease in a city and causing mass hysteria could be an excellent terrorist tactic. Likely better than a bomb that kills a few people.
From this point of view, mailing of anthrax in 2001 was very inefficient.
Numerous USA calamities make a lot more sense when we study them with an eye toward who benefits rather than who is blamed.
The anthrax mailing caused changes and fear in US society with very little work. That sounds like a very efficient terrorist attack IMO.