NEW YORK (TheStreet) -- The stock market is trying to price in the impact that Ebola will have if it continues to spread, Jim Cramer said Thursday on CNBC's "Squawk Box" show. Until there is more clarity on how to contain or treat the disease, the market will not be able to have a sustainable rally, he said.

Cramer, the co-manager of the Action Alerts PLUS portfolio, said an Ebola vaccine is not something that can be created overnight. It will take biotech companies some time to figure out a cure that works and is effective, he said.

Right now, "irrational fear" is gripping investors, Cramer said. Irrational fear is not useful for investors, but it will drive the market nonetheless, he added.

For example, Delta Air Lines (DAL) - Get Report reported a top- and bottom-line earnings beat on Thursday before the market opened. The company did not mention Ebola in its press release and should be positively impacted by lower jet fuel costs, but the stock is lower in premarket trading.

The only reason shares of Delta were trading lower is because of Ebola fears, Cramer said. 

Until there is some "stability" with the Ebola situation, the market will remain volatile as it continues to trade with irrational fear, Cramer concluded. 

At the time of publication, Action Alerts PLUS, which Cramer co-manages as a charitable trust, had no positions in stocks mentioned.

-- Written by Bret Kenwell.

Follow @BretKenwell