Chemical agents are relatively cheap and easy to produce in the same factories that make pesticides and fertilizer. So a chemical program is easy to conceal.
Mustard gas and chlorine gas were used in WWI, but the effects were so horrible that chemical weapons were banned in the 1925 Geneva Treaty. However, both sides used them in the 1980-88 Iran-Iraq war. They were banned again in the 1992 Chemical Weapons Convention, but a small fringe terrorist group in Japan carried out a nerve gas attack in the Tokyo subway in 1995, killing dozens of people. Fortunately, they were not able to master the relatively difficult task of effectively dispersing the gas beyond a small area. In the current war in Syria, the government has repeatedly used nerve gas, and more recently chlorine, to kill hundreds of people.
Biological agents are also now relatively cheap and easy to make, and can spread diseases far beyond the initial attack. Of course, the diseases could spread to the attacking country or even the whole world, as in the app Plague Inc. and the movies Rise of Planet of the Apes, Contagion and 12 Monkeys. Biological weapons were used by the Japanese in China in WWII. (Unit 731 killed about 2 million Chinese.) Also, in spite of the 1972 Biological Weapons Convention, the USSR built a huge secret program that produced massive amounts of anthrax, smallpox and other diseases, including some that were bioengineered to be worse than normal. (One accident resulted in anthrax deaths downwind.) None were ever used and the stocks were supposedly destroyed. There was a small powdered anthrax attack by mail in the U.S. soon after 9/11, but nothing since.
A bioterror attack is still a big worry, especially as advanced biotechnology techniques such as CRISPR get cheaper and more widespread. Recent threat assessments say that a biological attack is the most likely WMD threat and that a naturally-occurring epidemic of antibiotic-resistant superbugs is a serious civil defense threat. When a biologist recently engineered a more infectious version of a dangerous strain of flu, he was asked not to publish all the data, so that terrorists could not use it. Let’s hope that no rogue IS biologist makes a new flu or smallpox-plus.