The number of people who caught tuberculosis last year inched downward according to an estimate by the World Health Organization, but the agency warned that drug-resistant strains are still spreading.

In a new report issued Wednesday, the U.N. agency estimated there were about 8.7 million new cases of TB last year, down from about 8.8 million in 2010. The number of deaths was unchanged at about 1.4 million — making it the second-leading killer among infectious diseases after AIDS.

But no one knows for sure what the actual figures are since the WHO report said it was too expensive and complicated to measure the exact number of new TB cases every year.

WHO also said drug-resistant tuberculosis was spreading but acknowledged it didn't have enough data to know if those strains were getting more prevalent or not.

Drug-resistant TB is often the result of patients not being treated properly for regular TB; it is more expensive to treat and the drugs have worse side effects. WHO estimates that only 1 in 5 cases of drug-resistant TB are identified globally, meaning the others are spreading the disease without being treated.

A look at various regions:

— EASTERN EUROPE AND CENTRAL ASIA: These regions have the world's highest rates of drug-resistant TB, including 62 percent of previously treated patients in Uzbekistan and 75 percent of such cases in Belarus. Much of the disease's spread here is driven by intravenous drug users and weak health systems that don't identify and treat patients early enough.

— INDIA AND CHINA: These two countries have almost 40 percent of the world's TB cases. India and China also have a rising number of drug-resistant cases even though both countries claim to treat about 90 percent of their TB patients. Last year, India reported several cases of totally resistant TB that were untreatable.

— AFRICA: About 25 percent of the world's TB cases are in Africa, where the death rate is the highest in the world. The HIV epidemic is also fueling the spread of TB in sub-Saharan Africa. Patients with HIV often have weaker immune systems, making them more susceptible to catching TB. At least one-third of HIV patients also have TB and about one-quarter of deaths in people with HIV are due to TB.

— AMERICAS, MIDDLE EAST AND WESTERN EUROPE: While the fewest number of TB cases are found here, the disease is on the rise in some cities including London, due largely to global travel patterns. The bacteria are spread easily in the air and people need only to inhale a few of the germs to be infected.