Biological warfare is as fun and cheery a topic as nuclear war.

Weaponized disease is usually chosen on the grounds of its inability to keep spreading. You don't want to create a scenario where your weapon starts to kill your own people. In order to accomplish this, it requires weaponizing a suitable agent (say anthrax) and then dispersing it over your targets. This requires a lot more bombs and passes with aircraft or lobbing shells or however you're delivering it.

Not everyone in any given area is going to get sick and die as a result of biological weapons (again, using anthrax as an example). Given a marginal exposure, it's more likely to kill you the sicker you already are (due to pre-existing famine and war), or given that means of treatment are unavailable (likewise, due to war).

Granted you'll cause a lot of unrest and you'll kill off a lot of people, but there will be way too much clean up involved to make it worthwhile compared to conventional or even nuclear warfare. If you're going to go the biological route, get creative and attack crops and livestock. It won't be as easily recognized as a WMD (thus lessening their ability to gain international sympathy) but it may kill just as many as directly sickening the target population.

Like I keep saying, famine is always the killer. It's not flashy and it doesn't look good in a military parade, but it works.