You will be presented with the full text of a biomedical research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
this study was based on data from the healthcare cost and utilization project national inpatient sample ( hcup nis ) for the 5-year period 20002004 .
hcup nis is a stratified random sample from 20% of all us community hospitals . for 2004 ,
the nis contains all discharge data from > 1,000 acute - care hospitals in 37 states , representing > 8 million hospital stays .
we identified all hospital admissions in the hcup nis for which a principal diagnosis of ssti was given .
as defined by the uniform hospital discharge data set , principal diagnosis is the condition  established after study to be chiefly responsible for occasioning the admission of the patient to the hospital for care  ( http://wonder.cdc.gov ) .
sstis were defined as the following : 1 ) acute lymphadenitis ( international classification of diseases , 9th revision , clinical modification [ icd-9-cm ] 683.xx ) ; 2 ) carbuncle and furuncle ( 680.xx ) ; 3 ) cellulitis and abscess of finger and toe ( 681.xx ) ; 4 ) impetigo ( 684.xx ) ; 5 ) infection ( chronic ) of amputation stump ( 997.62 ) ; 6 ) other cellulitis and abscess ( 682.xx ) ; 7 ) other local infections of skin and subcutaneous tissue ( 686.xx ) ; 8) abscess of anal and rectal regions ( 566 ) ; 9 ) chronic ulcer of other specified sites ( 707.8 ) ; 10 ) chronic ulcer of unspecified site ( 707.9 ) ; 11 ) decubitus ulcer ( 707 ) ; 12 ) infection due to other vascular device implant and graft ( 996.62 ) ; 13 ) pilonidal cyst with abscess ( 685 ) ; 14 ) postoperative wound infection ( 998.5x ) ; 15 ) posttraumatic wound infection , not elsewhere classified ( 958.3 ) ; 16 ) ulcers of lower limbs , except decubitus ( 707.1x ) ; 17 ) gangrene ( 785.4 ) ; and 18 ) necrotizing fasciitis ( 728.86 ) . to aid in interpretation of the data , we grouped the above - listed sstis into 3 mutually exclusive categories : 1 ) superficial infections predominantly caused by s. aureus or streptococcus pyogenes ( groups 17 above ) ; 2 ) ; deeper or healthcare - associated infections more likely to involve anaerobic or gram - negative organisms ( groups 816 ) ; and 3 ) infections typically associated with a high rate of mortality ( groups 1718 ) .
for each year of interest , all ssti admissions were then stratified by type of infection and selected patient and hospital characteristics . to provide a benchmark against which to interpret possible trends in ssti - related hospital admissions
, we generated similar series for all admissions with a principal diagnosis of infectious pneumonia , another common type of infection that frequently results in hospitalization but for which no trends were expected .
infectious pneumonia was defined to include 1 ) pneumococcal pneumonia ( icd-9-cm 481 ) ; 2 ) other bacterial pneumonia ( 482.xx ) 3 ) ; pneumonia caused by other specified organism ( 483.x ) 4 ) ; bronchopneumonia , organism unspecified ( 485 ) ; and 5 ) pneumonia , organism unspecified ( 486 ) .
analyses were directed at discerning possible trends in ssti - related hospital admissions at us acute - care hospitals over the 5-year period of study .
total ssti admissions were estimated for each year by type of infection ( i.e. , superficial , deeper or healthcare - associated , often - fatal ) .
results also were stratified according to patient ( e.g. , age , sex ) and hospital characteristics ( e.g. , urban vs. rural ) available in the hcup nis . for comparison
, we also examined total hospital admissions for infectious pneumonia . because the hcup nis undergoes periodic changes in its sampling frame and weighting method , as well as in data elements and definitions
, we used the nis trends supplemental ( nis - trends ) files to ensure comparability of data across the years of interest .
the nis - trends files contain sampling weights and data elements that are consistently defined across all years of interest .
analyses were primarily descriptive ( i.e. , no a priori hypotheses were made , and our results were not subjected to formal significance testing ) .
all analyses were conducted by using the sas proprietary software , release 9.1 ( sas institute , cary , nc , usa ) .
the estimated total number of annual ssti admissions to us acute - care hospitals rose steadily over the 5-year period , from 675,000 in 2000 to 869,800 in 2004 , an increase of 194,000 ( 29% ) admissions .
in contrast , total admissions for infectious pneumonia fluctuated from year to year and were largely unchanged over this period ( table 1 ) .
the increase in ssti admissions was greatest among younger ( age < 65 years ) rather than older patients ( age 65100 years ) ( 37% vs. 14% , respectively ) and for urban rather than rural hospitals ( 32% vs. 11% ) ( table 2 ) .
the increase in ssti admissions also was greatest among patients with superficial infections ( e.g. , cellulitis , abscess ) rather than deeper or healthcare - associated infections ( e.g. , postoperative wound infection , infection due to vascular device ) ( 33% vs. 24% ) .
* ssti , skin and soft tissue infection ; icd-9-cm , international classification of diseases , 9th revision , clinical modification ; nec , not elsewhere classified .
until recently , ca - mrsa was relatively uncommon . in 2000 , for example , these isolates accounted for only 3% of staphylococcal isolates submitted to minnesota laboratories ( 4 ) . in recent years , however , ca - mrsa has become a major cause of sstis .
the prevalence of mrsa among patients with sstis who sought treatment at 1 los angeles area emergency department increased from 29% in 2001 to 64% in 2004 ( 5 ) .
while the estimated total number of ssti admissions to us acute - care hospitals increased by 29% during 20002004 , admissions for infectious pneumonia were largely unchanged .
we are unaware of any other data to which our findings might be directly compared .
our results appear to be consistent , however , with those of previous studies that have reported an increasing prevalence of illness attributable to s. aureus infections in general and to mrsa in particular ( 6,7 ) .
although we could not establish in our study whether the increase in the number of ssti hospital admissions was a result of the growing prevalence of ca - mrsa ( the hcup - nis does not report microbiologic data ) , we suspect that the 2 phenomena are closely linked  especially in light of the absence of any similar increase in hospital admissions for pneumonia , the most common community - associated infection requiring hospitalization .
we therefore believe that the clinical and economic effects of ca - mrsa sstis are substantial and growing , and that this increase should be a focus of additional research .
using data from the 20002004 us healthcare cost and utilization project national inpatient sample , we found that total hospital admissions for skin and soft tissue infections increased by 29% during 20002004 ; admissions for pneumonia were largely unchanged . 
 these results are consistent with recent reported increases in community - associated methicillin - resistant staphylococcus aureus infections .